MjH ELECTRO
MECHANISMS
MOTOR
CONTROLS
Electromechanical
Technology Series
TERC EMT STAFF
DELMAR
PUBLISHERS, MOUNTAIN VIEW AVENUE, ALBANY, NEW YORK 12205
WITHDRAWN
TK ZS51 -«.s 11,71
SHt»""e*»»' ,is " s * n0T0 ''
Anderson, D A
TK
2851
* A65 Electro mechanisms; motor
1971 controls
001
[1971]
; YST j;
UNIVERSITY LIBRARY
WESTERN KENTUCKY UNIVERSITY
BOWLING GREEN, KENTUCKY 42101
1 2/12/74 00 I
45679-
ELECTRO
MECHANISMS
MOTOR
CONTROLS
WEST. KY. UNIV. LIB.
DELMAR PUBLISHERS, MOUNTAINVIEW AVENUE. ALBANY, NEW YORK 12205
.CHANTS^ noTO*
DELMAR PUBLISHERS
Division of Litton Education Publishing, Inc.
Copyright (T) 1971"
By Technical Education Research Centers, Inc,
Copyright is claimed until December 1, 1976. There-
after all portions of this work covered by this copy-
right will be in the public domain.
All rights reserved. No part of this work covered by
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U.S. Office of Education, and no official endorsement
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74 - 170789
Foreword
The marriage of electronics and technology is creating new demands for
technical personnel in today's industries. New occupations have emerged
with combination skill requirements well beyond the capability of many
technical specialists. Increasingly, technicians who work with systems and
devices of many kinds — mechanical, hydraulic, pneumatic, thermal, and
optical — must be competent also in electronics. This need for combination
skills is especially significant for the youngster who is preparing for a career
in industrial technology.
This manual is one of a series of closely related publications designed
for students who want the broadest possible introduction to technical occu-
pations. The most effective use of these manuals is as combination textbook-
laboratory guides for a full-time, post-secondary school study program that
provides parallel and concurrent courses in electronics, mechanics, physics,
mathematics, technical writing, and electromechanical applications.
A unique feature of the manuals in this series is the close correlation of
technical laboratory study with mathematics and physics concepts. Each
topic is studied by use of practical examples using modern industrial applica-
tions. The reinforcement obtained from multiple applications of the concepts
has been shown to be extremely effective, especially for students with widely
diverse educational backgrounds. Experience has shown that typical junior
college or technical school students can make satisfactory progress in a well-
coordinated program using these manuals as the primary instructional material.
School administrators will be interested in the potential of these
manuals to support a common first-year core of studies for two-year
programs in such fields as: instrumentation, automation, mechanical design,
or quality assurance. This form of technical core program has the advantage
of reducing instructional costs without the corresponding decrease in holding
power so frequently found in general core programs.
This manual, along with the others in the series, is the result of six years
of research and development by the Technical Education Research Centers,
Inc., (TERC), a national nonprofit, public service corporation with head-
quarters in Cambridge, Massachusetts. It has undergone a number of revisions
as a direct result of experience gained with students in technical schools and
community colleges throughout the country.
Maurice W. Roney
Hi
The Electromechanical Series
TERC is engaged in an on-going educational program in Electromechani-
cal Technology. The following titles have been developed for this program:
INTRODUCTORY
ELECTROMECHAN ISMS/ MOTOR CONTROLS
ELECTROMECH AN ISMS/DEVICES
ELECTRONICS/AMPLIFIERS
ELECTRONICS/ELECTRICITY
MECHANISMS/DRIVES
MECHAN ISMS/LI N KAGES
UNIFIED PHYSICS/FLUIDS
UNIFIED PHYSICS/OPTICS
ADVANCED
ELECTROMECHAN ISMS/AUTOMATIC CONTROLS
ELECTROMECHAN ISMS/ SERVOMECH AN ISMS
ELECTROMECHANISMS/FABRICATION
ELECTROMECHAN ISMS/TRANSDUCERS
ELECTRONICS/COMMUNICATIONS
ELECTRONICS/DIGITAL
MECHANISMS/MACHINES
MECHANISMS/MATERIALS
For further information regarding the EMT program or for assistance in
its implementation, contact:
Technical Education Research Centers, Inc.
44 Brattle Street
Cambridge, Massachusetts 02138
iv
Preface
Technology, by its very nature, is a laboratory-oriented activity. As
such, the laboratory portion of any technology program is vitally important.
These materials are intended to provide meaningful experience in electronic
motor control for students of modern technology.
The topics included provide exposure to basic principles of motor con-
trol, SCR controls for various types of motors, and an introduction to digital
controls.
The sequence of presentation chosen is by no means inflexible. It is
expected that individual instructors may choose to use the materials in
other than the given sequence.
The particular topics chosen for inclusion in this volume were selected
primarily for convenience and economy of materials. Some instructors may
wish to omit some of the exercises or to supplement some of them to better
meet their local needs.
The materials are presented in an action-oriented format combining
many of the features normally found in a textbook with those usually asso-
ciated with a laboratory manual. Each experiment contains:
1. An INTRODUCTION which identifies the topic to be examined
and often includes a rationale for doing the exercise.
2. A DISCUSSION which presents the background, theory, or tech-
niques needed to carry out the exercise.
3. A MATERIALS list which identifies all of the items needed in the
laboratory experiment. (Items usually supplied by the student
such as pencil and paper are not normally included in the lists.)
4. A PROCEDURE which presents step-by-step instructions for per-
forming the experiment. In most instances the measurements are
done before calculations so that all of the students can at least
finish making the measurements before the laboratory period ends.
5. An ANALYSIS GUIDE which offers suggestions as to how the
student might approach interpretation of the data in order to
draw conclusions from it.
6. PROBLEMS are included for the purpose of reviewing and rein-
forcing the points covered in the exercise. The problems may be
of the numerical solution type or simply questions about the
exercise.
v
Students should be encouraged to study the text material, perform the
experiment, work the review problems, and submit a technical report on
each topic. Following this pattern, the student can acquire an understanding
of, and skill with, basic motor control circuits that will be extremely valu-
able on the job. For best results, these students should be concurrently
enrolled in a course in technical mathematics (analytic geometry and intro-
ductory calculus).
These materials comprise one of a series of volumes prepared for tech-
nical students by the TERC EMT staff at Oklahoma State University, under
the direction of D.S. Phillips and R.W. Tinnell. The principal author of these
materials was D.A. Anderson.
An Instructor's Data Guide is available for use with this volume.
Mr. R.C. Davidson and Kenneth F. Cathy were responsible for testing the
materials and compiling the instructor's data book for them. Other mem-
bers of the TERC staff made valuable contributions in the form of criti-
cisms, corrections, and suggestions.
It is sincerely hoped that this volume as well as the other volumes in
this series, the instructor's data books, and the other supplementary mate-
rials will make the study of technology interesting and rewarding for both
students and teachers.
THE TERC EMT STAFF
TO THE STUDENT
Duplicate data sheets for each experiment are provided in the back of
the book. These are perforated to be removed and completed while perform-
ing each experiment. They may then be submitted with the experiment
analysis for your instructor's examination.
vi
Contents
experiment 1 SI LICONE-CONTROLLED RECTIFIERS 1
experiment 2 DC MOTOR CONTROL 7
experiment 3 DC CONTROL OF AN AC MOTOR 13
experiment 4 AC AND DC VOLTAGES FOR SCR MOTOR CONTROL . ... 24
experiment 5 PHASE SHIFT CONTROL CIRCUITS . . ' 31
experiment 6 PHASE SHIFT CIRCUIT FOR SCR MOTOR CONTROL .... 38
experiment 7 THE UNIJUNCTION TRANSISTOR 46
experiment 8 UNIJUNCTION TRANSISTOR FOR SCR CONTROL 51
experiment 9 MOTOR CONTROL BY PULSE HEIGHT VARIATION .... 59
experiment 10 DIGITAL MOTOR CONTROL 65
experiment 1 1 THE TRIAC FOR AC MOTOR CONTROL 72
experiments SYNCHRONOUS MOTOR CONTROL 78
experiment 13 INCREMENTAL MOTORS 84
experiments TACHOMETER FEEDBACK 90
experiment 15 SERVOMECHANISMS 98
Experiment Data Sheets Back of Book
vii
experiment SILICON-CONTROLLED RECTIFIERS
INTRODUCTION. Throughout the growth and development of two-layer (diode, PN junction)
and three-layer (transistor, PNP or NPN) semiconductor devices, the evolution of a four-layer
device has been imminent. The silicon-controlled rectifier (SCR) is a four-layer, three-terminal
device which has become very popular for use in many types of circuits. Its efficiency, rugged-
ness, and compactness make it particularly useful in current and voltage control circuits. This
exercise is an introduction to the theory, parameters, and ratings of the silicon-controlled
rectifier.
DISCUSSION. Since about 1957 when the
SCR was introduced, this unique component
has carved a niche for itself in military, indus-
trial, commercial and residential applications.
In hundreds of different jobs, it has replaced
thyratrons, relays, magnetic amplifiers and
saturable reactors. It is used in controllers,
switches, and timers. But more important
than the replacing of existing components,
the SCR has made possible many new prod-
ucts, hitherto unfeasible. Among them are
ultrahigh-speed protective devices and light-
weight, compact, power controls.
Although the silicon-controlled rectifier
may be thought of as a solid-state thyratron,
its forward voltage drop is about one-tenth
that of a gas thyratron, making it much more
efficient.
j t I j 2
i .. i
Fig. 1- 1 Physical Arrangement of a Four-
Layer Device
An NPNP silicon-controlled rectifier is a
solid-state semiconductor device composed of
four layers of alternate conductivity-type
semiconductor material. This physical con-
struction and three junctions are shown in
figure 1-1.
In order to demonstrate the effect of the
three junctions on each other, a two- transistor
analogy can be used as shown in figure 1-2.
Fig. 1-2 Two-Transistor Analogy of a Four- Layer Device
1
EXPERIMENT 1 SILICON-CONTROLLED RECTIFIERS
MOTOR CONTROLS
An analysis of the PNPN device can be made
in terms of the parameters of the two transis-
tors in the analogy. The two transistors have
normal operating bias (forward bias) on J-|
and J3 while J2 is reverse-biased. Each tran-
sistor has a current gain (a) associated with
it. The current gain of the NPN transistor
will be called a-|, and the current gain of the
PNP transistor will be called a^. Since a is,
by definition, that fraction of the current in-
jected at the emitter that reaches the collector,
the operation of the two devices can be com-
bined to explain the action of the controlled
rectifier. The collector junction for both
transistors is J2; and, in the NPNP device, J2
is affected by three components of current as
shown in figure 1-3.
The current in the external circuit must
pass through J2, so
'J2= 1
or
or
'j2 =l=a 1 l+a 2 l + l co
Then,
(1.1)
M1- a 1- a 2> = l co
which gives
I _ a, I - 0£2l = l co
(1-2)
From the above expression, it can be seen
that, as a-| + 03 approaches unity, the current
through the device becomes large and is lim-
ited only by the resistance of the external
circuit.
As previously mentioned, J-| and J3 are
forward-biased, and J2 is reverse-biased. Since
J2 is reverse-biased, the initial current through
the device may be very small.
The l co of a silicon device can be made
very small and if a<\ and 03 are q u ' te small,
equation 1.2 shows that the total current I in
the device will also be very small. This condi-
tion is said to be the "off" state of the device.
The "on" condition of the device exists when
ce-j + 0C2 approach unity. If a-| + 0L2 = 1, the
current through J2 and the external circuit is
limited only by the resistance of the external
circuit.
«l'
l J2 = a, I +a 2 l + l c y— (LEAKAGE CURRENT)
I t (HOLE CURRENT FROM END P REGION)
I (ELECTRON CURRENT FROM END N REGION)
Fig. 1-3 Approximate Currents Flowing in the Four-Layer Device
2
MOTOR CONTROLS
EXPERIMENT 1 SILICON-CONTROLLED RECTIFIERS
Fig. 1-4 Variation of a with VQ E and l Q
According to transistor theory, there are
several ways of increasing the current gain (c^
and c^) of the component junction transistors
^ , j
to turn "on" the SCR. Figure 1-4 shows two
of the ways of increasing current gain.
As shown in figure 1-4A, the current
gain, a, increases slightly as the collector-to-
emitter voltage is increased until a value of
Vqe ' s reac ' iec ' where the energy of the car-
riers arriving at the collector PN boundary is
sufficient to dislodge additional carriers pro-
ducing a form of avalanche breakdown. This
causes a to increase much more rapidly with
voltage.
When this avalanche breakdown occurs
at J2, the current through J2 increases which,
in turn, increases ai and 0.2* causing the de-
vice to go into its "on" state. After the de-
FORWARD BIAS
vice has gone into its "on" state, it will remain
in this state so long as the current through J2
is sufficient to cause a-j + 0:2 to be near unity.
The value of current through J2 required to
keep the device in the "on" condition is
known as the holding current, l|_|. Figure 1-5
shows the forward bias El curve for a PN
junction and for a PNPN device. Notice that
in figure 1-5 the El curve for the four-layer
device is similar to the two-layer once it is in
the "on" condition.
In most typical silicon transistors, a is
quite small at low emitter currents but in-
creases fairly rapidly as the emitter current is
increased. This effect, shown in figure 1-4B,
is due to the presence of impurities in the
silicon. In order to increase the emitter cur-
rent in the four-layer device independent of
the voltage across it, a connection must be
p
N
P
N
FORWARD BIAS
B RE A KO VE R
HOLDING CURRENT /CURRENT
PN El PLOT PNPN El PLOT
Fig. 1-5 Forward Bias El Plots for PN Junction and PNPN Junction
FORWARD
BREAKOVER
VOLTAGE
3
EXPERIMENT 1 SILICON-CONTROLLED RECTIFIERS
MOTOR CONTROLS
Gate
r
p
N
P
N
Fig. 1-6 Gate Connection of SCR to Cause
Turn-On
made to the base of one of the transistor
sections. This terminal is known as the gate
of the SCR and is shown in figure 1-6.
Figure 1-7 shows the change in forward
breakover voltage with different values of gate
current. As the gate current is increased, the
breakover voltage decreases. Once the device
is turned "on", the only way to turn it "off
is to reduce the voltage across the device
which will, in turn, reduce the current through
I
E
ZENER
BREAKDOWN
; 2 >l G1 >l G = 0
•W 'til ' 'ta
r — — ? i *
i G2 >i g1 >o
Fig. 1-7 Effect of Increasing Gate Currents
on SCR Characteristics
it until it is below the minimum value of the
holding current. The gate no longer has any
effect while the device is "on."
In the reverse direction, both devices
have similar El curves as shown in figure 1-8.
+
N
P
N
P
REVERSE BIAS
Fig. 1-8 Reverse Bias El Plot for Both the PN and the PNPN Junctions
MATERIALS
1 SCR, type CE1 06 or equiv.
2 Variable DC power supplies, 0-40V
2 Resistance decade boxes (0-100k£2 2W)
2 Multimeters
1 Oscilloscope
1 Variable AC voltage source
4
MOTOR CONTROLS
EXPERIMENT 1 SILICON-CONTROLLED RECTIFIERS
PROCEDURE
1. Connect the SCR circuit shown in figure 1-9. For a type CE106 SCR, Ri = 1kfi,
R 2 = 2.2kn.
CAUTION: Check the maximum ratings of the SCR. Make fl; large enough
to limit the anode current to a safe value for both the power supply and SCR
at the maximum voltage to be used. Make Rg large enough to limit the gate
current to a safe value. Check the wattage needed for both resistances.
R 1
+
A
sen
^— vv\ — (T) 1 _
POWER fr
SUPPLY V
POWER
SUPPLY
l +
Fig. 1-9 Circuit for Obtaining Some of the SCR Characteristics
2. Disconnect the gate circuit and set the anode voltage to about 30V. Measure the current
through the SCR. ("Off" or leakage current, l F0 ).
3. Connect the gate circuit and set the cathode current meter to a high scale (value expected
when SCR is on). Increase the gate current until the SCR conducts as indicated by a sharp
increase in anode current. Record the gate current (\q) required to turn the SCR "on".
Remember that once the SCR has fired, the only way to turn it off is to reduce the anode
current below the holding value.
4. Measure the voltage across the SCR (V^k) and determine the forward "on" resistance
<Ron>-
5. Reduce the gate current to zero. Slowly reduce the anode voltage and watch the drop in
anode current. Minimum holding current will be the value reached when anode current
drops abruptly to zero. Record its value, I n«
6. Disconnect the gate circuit and reverse the cathode and anode connections. Apply about
30V to the SCR and measure the reverse leakage current (I rq).
5
EXPERIMENT 1 SILICON-CONTROLLED RECTIFIERS
MOTOR CONTROLS
R 1 AND R 2 SAME AS BEFORE.
VARIABLE
AC
SOURCE
VARIABLE
AC
POWER SUPPLY
Fig. 1-10 Circuit for Observing SCR Operation
7. Connect the SCR in a circuit similar to figure 1-10.
CAUTION: Be careful in connecting the circuit and the common leads of the
test instruments. An incorrect connection can damage the SCR.
8. Adjust the AC source for about 80V RMS and observe the anode-cathode waveform
while adjusting the gate current.
9. Set the gate current to trigger the SCR in the mid-portion of the sine wave and vary the
AC anode voltage.
Fig. 1-11
The Data Table
'fo
'g I
V CE
R on
'H
'ro
PROBLEMS
1 . Determine the forward "off" resistance of the SCR.
2. Determine the forward "on" resistance of the SCR.
3. Determine the reverse resistance of the SCR.
4. How would you describe the efficiency of the SCR?
5. How would you compute a gain factor for the SCR?
6. How would the SCR react if excessive voltage is applied in the forward direction?
7. What would happen if excessive voltage were applied in the reverse direction?
8. Compare the SCR ("on" condition) with a silicon diode.
9. Describe what was observed in step 8 and 9.
6
experiment X DC MOTOR CONTROL
INTRODUCTION. There are many applications of DC motors in which speed control is desirable.
The SCR is a small, efficient, and relatively inexpensive device that can be used to control a DC
motor from an AC source. This experiment is an introduction to the use of an SCR for DC motor
speed control.
DISCUSSION. One important advantage of
the SCR as a control device is that it has very
low leakage currents and low forward resis-
tance when in the "on" condition. It is,
therefore, quite efficient. Also, the amount
of gate current required to turn the device on
is small with respect to the current the device
will handle in the "on" condition. In other
words, a very small current can be used to
control a much larger current.
Before discussing the control circuit, DC
motor principles will be reviewed briefly. A
DC motor consists of a field, armature, com-
mutator and brush assembly as shown in
figure 2-1 . This basic motor can be connected
in several ways. First of all, if the field
strength is to be constant, it can be replaced
by a permanent magnet and power applied
to the armature only. In the case of an elec-
tromagnetic field, figure 2-2 shows some typi-
cal connections.
FIELD (POLES - WINDING)
Fig. 2- 1 Components of the DC Motor
SEPARATE
EXCITATION
SERIES
SHUNT
COMPOUND
Fig. 2-2 Connections for DC Motors
7
EXPERIMENT 2 DC MOTOR CONTROL
MOTOR CONTROLS
FIELD
ARMATURE
Fig. 2-3 Equivalent Circuit for Separately- Excited DC Motor
The expression for the armature voltages
present in the separately-excited DC motor
may be written from the equivalent circuit in
figure 2-3.
E dc" 'dc R a + E a
E a =K 1 0 f w
(2.1)
(2.2)
where
E 0 = counter emf of the armature
d
in volts
0f = effective field flux in webers
co = speed in RPM
FL = armature resistance in ohms
d
K-| = a proportionality constant
l dc = armature current in amps
Substituting equation 2.2 into 2.1 gives
E dc-'dc R a + K 1*f"
Solving this equation for to gives
K 1 0 f co = E dc -l dc R a
CO =-
E dc" 'dc R a
1*f
From equation 2.3 we can see that the speed
of the motor can be controlled by controlling
the applied voltage E^.
The torque of the motor is given by the
expression
T= Mf'dc
(2.4)
(2.3)
where T = torque in in.-oz
0f = effective field flux in webers
l^c = armature current in amps .
K2 = constant
From this expression we notice that the cur-
rent will vary with the torque produced at a
given speed.
Other types of motor connections will
react somewhat differently because of the
interaction of the field with the applied volt-
age, but their speed can also be controlled by
controlling the applied voltage.
Figure 2-4 shows a simple diode circuit
with a resistive load. A DC motor could be
substituted for the resistive load in this cir-
cuit.
8
MOTOR CONTROLS
EXPERIMENT 2 DC MO TOR CONTROL
RL
Fig. 2-4 Halfwave Rectifier Circuit
When the generator voltage (V g ) under- Since the rectifier produces pulses of DC from
goes its positive excursion, the diode conducts an AC source, the average DC current can be
and the voltage distribution of the circuit is found by
Vg = V b + V R| _
or
v g =i b r b + i b R L =i b< r b +R L>
(2.5)
where
Vg = instantaneous generator
voltage
= diode anode-to-cathode
voltage
i^ = diode forward current
r^ = forward resistance of the diode
When Vg undergoes its negative excursion, V^
is negative and the diode does not conduct
except for the leakage current which is very
small. Therefore, ibR|_ anc ' Vrl are nearly
zero. If the generator voltage is
Vg = E m sin cot
then
and
i b = 0
■m
l =—□.
dc ttR
7T
(2.6)
The average DC voltage across the load will be
(2.7)
I R - p -![!^-£m
l dc H L _t dc" 7T " 7T
7r <cot <2ir
The effective value of current (l rms ) can be
found from its definition
(2.8)
The power delivered to the entire circuit by
the generator is
P= ('rms) 2 < R L + r b) < 2 - 9 >
and the average power delivered to the load is
PRL,av=<'rms> 2R L = <T )2R L (2 ' 10)
9
EXPERIMENT 2 DC MOTOR CONTROL
MOTOR CONTROLS
Fig. 2-5 Waveforms in a Motor Supplied by a Controlled Rectifier
Figure 2-5 shows the waveforms of the
voltage and current supplied to the armature
of a DC motor using an SCR to allow less than
1/2 cycle of conduction. The motor has a
separately-excited field. Just before the in-
stant of firing at 0<|, the current is zero and
the armature voltage is equal to the counter
emf, E a . At 0<| the SCR fires and the arma-
ture voltage jumps to e-|. The armature cur-
rent then builds up slowly because of the
armature inductance, and continues to 02,
when conduction ceases and the voltage is E a .
Notice that the net voltage available to
build up current in the circuit is e g - E a or
E m sin cot - E a . During the interval from 0<[
to 0 X , at which time E m sin cot = E a , the
applied potential can produce a positive cur-
rent in the circuit. The remainder of the con-
duction period, from 0 X to 02, is due to the
L(di/dt) potential arising from the decreasing
10
MOTOR CONTROLS
EXPERIMENT 2 DC MOTOR CONTROL
current in the armature inductance. With
triggering occurring at 0j, the direct current
produced by a pulse like the one in figure 2-5
can be found by
-m
(2.11)
MATERIAL'S
1 28V DC motor
1 Variable AC source
1 DC power supply 0-40V
1 SCR, Type CE106 or equivalent
As 0 increases from 0 to 180°, the cos 6 will
change from 1 to -1, so the value of (1 +
cos 0) will decrease from 2 to 0. It can be
seen from equation 2.11 that as 0 increases,
the average DC decreases, which, in turn, will
decrease the average voltage and power. Thus,
changing the gate current of the SCR changes
0 and will result in a change in motor speed.
1 Resistor, 470fi 2W
1 Oscilloscope
1 Multimeter
1 Stroboscope
PROCEDURE
1 . Construct the circuit shown in figure 2-6 with all voltages set to zero.
CAUTION: Make sure that the common terminal of the DC power supply is
connected to the common side of the AC power supply.
VARIABLE
DC SUPPLY
Fig. 2-6 Circuit for Controlling a DC Motor
2. Set the AC voltage to about 30V. With no DC voltage on the gate circuit, the motor
should not run.
3. Carefully increase the gate current by increasing the DC voltage until the motor begins
to run.
4. Measure the gate current (\q) that causes the motor to start running, the average DC
voltage across the motor measured with a DC voltmeter (V m ), and the firing angle of the
SCR observed on an oscilloscope (0-|).
5. Increase the DC control voltage until the firing angle is zero and the DC voltage across
the motor stops increasing. Record the motor voltage, motor speed, and gate current.
EXPERIMENT 2 DC MOTOR CONTROL
MOTOR CONTROLS
6. Make about ten readings of gate current (Iq), motor voltage (V m ), motor speed (co), and
firing angle of the SCR (8<\) between the limits of steps four and five.
7. Plot a graph of each function with respect to each of the other functions.
Conditions
'G
V m
0 1
CO
Start
0., = 0
Run
Fig. 2-7 The Data Table
ANALYSIS GUIDE. The DC motor is a very popular and useful energy converter and is a typical
electrical/mechanical transducer. The DC motor becomes even more useful when it can be speed-
controlled by a low-power control signal. In this exercise a very simple method of SCR motor
control that has a distinct disadvantage has been used. You should become aware of this disad-
vantage as you perform the experiment. Discuss this disadvantage in your own words.
PROBLEMS
1. With respect to control power, is the SCR an efficient means of control?
2. Is Figure 2-6 a satisfactory control circuit? Discuss.
3. What characteristic of the SCR is responsible for your conclusion in question 2?
4. How much change in gate current is required to change the speed from just starting
to full speed?
5. Show the change in waveform as the motor speed is increased.
12
experiment
3
DC CONTROL OF AN AC MOTOR
INTRODUCTION. In many applications of AC motors it is desirable or necessary to regulate or
control the speed. Because of its compactness, efficiency, ease of control and switching speed,
the SCR is very useful in motor control circuits. In this exercise a simple control circuit employ-
ing two SCRs will be used to control an AC motor.
DISCUSSION. There are three main types of
AC motors used when speed control is em-
ployed:
Torque is produced by the force devel-
oped when a current flows through a con-
ductor in a magnetic field.
(a) AC commutator motors
F = BIC newtons
(b) Induction motors supplied at con-
stant frequency, but with some
means of controlling the stator or
rotor current
(c) Induction motors supplied at vari-
able frequencies from an alternator
or controllable rectifier.
You will recall that a DC motor has fixed
magnetic poles (stator) and rotating poles
(rotor). The rotating poles are energized by
commutator connections and may be in series
or parallel with the stationary fields, or the
motor may be separately excited. Figure 3-1
shows a DC motor with separate excitation.
where B is flux density in webers per
square meter
I is current through the con-
ductor in amperes
£ is the active conductor length
in meters
In figure 3-1 the direction of the force
exerted on the one-turn armature winding
(dotted lines) is indicated by the colored ar-
rows. As the armature rotates to the position
shown (solid lines), the commutator reverses
the direction of the current so that the direc-
tion of rotation remains the same. If the field
and armature currents are both reversed, the
direction of rotation still remains the same.
DIRECTION
OF
FORCE
ARMATURE WITH
COMMUTATOR
ARMATURE
FIELD
FIELD
(A) PICTORIAL DRAWING
(B) SCHEMATIC SYMBOL
Fig. 3- 1 Basic Parts of a DC Motor
13
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
MOTOR CONTROLS
AC MOTOR
o
AC SOURCE
SCRi
it
CONTROL
CKTS
SCR-
Fig. 3-2 SCR Circuit for Controlling an AC Motor
The direction of rotation will reverse when
either field or armature current is changed
with respect to the other. A commutated AC
motor or universal motor is basically the same
as a DC motor. With the field and armature
excited from the same voltage, a reversal in
voltage will reverse both and the torque will
continue in the same direction. Therefore, an
AC voltage will produce a varying torque but
always in the same direction.
Speed control of this type of motor can
be accomplished in the same manner as with
DC motors. An SCR circuit can be used to
control the average voltage to the motor from
a constant voltage source as shown in figure
3-2.
The main difference in an induction
motor and a universal motor is that the induc-
tion motor does not have a commutator or
other electrical connection from the external
circuit to the rotor. The rotor current is in-
duced from the changing magnetic field of the
stator so it must be excited with a constantly
varying voltage. A DC component will only
be wasted energy in an induction motor.
The basic principle of the induction
motor can be illustrated by using a permanent
magnet, iron plate and a copper or aluminum
disc as shown in figure 3-3. The field of the
permanent magnet is completed by the iron
plate. As the magnet rotates on the string,
the magnetic field is changing with respect to
any part of the disc. This action induces eddy
currents into the disc. The induced voltage
produced by the changing magnetic field (rota-
tion of the magnet) causes eddy currents and
they, in turn, produce another magnetic field.
According to Lenz's Law, the induced volt-
ages and resultant currents produce a field
tending to oppose the force or motion which
produced the induced voltage. As shown in
figure 3-3(B) the induced eddy currents tend
to produce a unit south pole in the disc at a
point under the rotating north pole of the
magnet, and a unit north pole in the disc under
the rotating south pole of the magnet. Since
opposite poles attract, the poles in the disc
will be attracted to the poles of the rotating
magnet and the disc will, therefore, rotate in
the direction of the permanent magnet rota-
tion. As long as the magnet moves with re-
spect to the disc, induced voltages will cause
eddy currents resulting in poles that are at-
tracted to the magnet's poles. If the disc were
to rotate at the same speed as the magnet,
there would be no relative motion, no induced
14
MOTOR CONTROLS
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
STRING
PERMANENT
MAGNET
DIRECTION OF
INDUCED EDDY
CURRENTS
BEARING &
PIVOT
(A) SIDE VIEW
COPPER OR
ALUMINUM
DISC
IRON PLATE
ROTATION
OF
MAGNET
ROTATION OF DISC
(B) TOP VIEW
Fig. 3-3 Induction Motor Principle
voltage to cause eddy currents, and no pole
to react with the magnet, so there would be
no force to turn the disc. The disc, therefore,
rotates in the same direction as the magnet,
but it must rotate at a speed less than that of
the magnet in order to produce a torque.
The rotor of a practical induction motor
is designed to be much more efficient than the
disc used in the above example. Figure 3-4
shows typical construction of a rotor for an
induction motor. The cage, which consists
of the end rings and the conductors, is usually
copper or aluminum and is imbedded in the
laminated iron core of the rotor. The rotor
has only a few low resistance conductors,
which allows high currents to produce strong
poles.
END
RINGS
CONDUCTORS
1
In all motors, while electromagnetic
torque is being produced as a result of the
interaction between the magnetic fields (rotor
and stator), generator action is simultaneously
occuring. In the AC induction synchronous
motor, the motor action and the generator
action occur at the synchronous speed of the
rotating magnetic field. In the AC induction
asynchronous motor, neither motor action
nor the generator action occurs at the syn-
chronous speed because of the necessary slip
required to produce the torque. For this rea-
son, induction principle machines are classed
as synchronous or asynchronous (non-syn-
chronous).
The synchronous type of motor, which
runs in synchronism with the line frequency,
IRON LAMINATIONS
Fig. 3-4 Construction of the Rotor of an Induction Motor
15
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
MOTOR CONTROLS
would have to be speed-controlled by chang-
ing the frequency of the applied voltage. This
type of speed control will not be discussed
here. As previously mentioned, the rotor of
the non-synchronous induction motor must
"slip" (in the above explanation, the disc did
not turn as fast as the magnet) in order to
produce torque. In a practical induction
motor there is a rotating magnetic field
caused by the applied AC voltage, the stator
poles, and their arrangement. This rotating
field replaces the magnet in our previous ex-
planation. The rotor will always have some
slip with respect to the rotating field. The
slip is often expressed as a percentage of the
effective speed of the rotating field and may
be found by
s = B
co 0 -co r X 100
or
where
w r = w o (1-s) = 120-(1-s)
s = Percent slip
oo Q = Synchronous speed (120f/P)
in RPM of the rotating stator
field
co r = Speed of the rotor in RPM
f = Frequency
P = Number of poles
Mr
LOW R
HIGH X
A "simple" single-phase winding will not
produce a rotating field and, consequently,
there will be no starting torque. Various
techniques are used to produce the rotating
field required for starting a single-phase induc-
tion motor. Fortunately an induction rotor,
once started, will continue to operate from a
single-phase supply. Since the field will re-
verse with every half cycle of line voltage, you
can consider that the field is rotating in half-
turn steps— in either direction. The armature
will, in fact, continue to rotate in the direction
in which it is started. Because a single-phase
induction motor is inherently not self-starting,
various methods are employed to initiate ro-
tation of the rotor. As a consequence, a clas-
sification system for single-phase induction
motors has emerged based on starting meth-
ods. A motor can be started by mechanically
putting the rotor in motion but nearly all
small motors have automatic starting devices
built into them. There are starting methods
that are electrical and others that are combi-
nation electrical and mechanical. Some of the
starting methods include split-phase resistance-
start, split-phase capacitor-start, permanent-
split single-value capacitor start, two-value
capacitor start, shaded pole induction, and
reluctance-start induction motor. Figure 3-5
shows some of these different starting tech-
niques.
CENTRIFUGAL
M CONNECTION DIAGRAM (o} PHASE RELATIONS
A. SPLIT-PHASE (RESISTANCE-START) INDUCTION MOTOR
Fig. 3-5 Different Starting Techniques
6
MOTOR CONTROLS
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
C. PERMANENT-SPLIT (SINGLE-VALUE) CAPACITOR MOTOR
CENTRIFUGAL
SWITCH
6
2
USE OF TWO CAPACITORS AND A
CENTRIFUGAL SWITCH
D. TWO-VALUE CAPACITOR MOTOR
Fig. 3-5 Different Starting Techniques (ContJ
17
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
MOTOR CONTROLS
(a) GENERAL CONSTRUCTION OF 2-POLE
SHADED-POLE MOTOR
The principal method of speed control
used for fractional horsepower single-phase,
induction-type, shaded-pole, reluctance, and
even series and universal motors is the method
of primary line voltage control. Figure 3-6
shows some typical torque slip curves. The
torque under starting and running conditions
varies as the square of the voltage impressed
on the stator (T = kV 2 p). For any value of
load, reducing the line voltage will reduce the
torque by the square of the reduction in line
voltage, and the reduction in torque will result
in an increase in slip, s. Reducing the line
voltage and the torque as a method of increas-
ing slip will serve to control the speed to a
(d) DECREASING 0
(b) INCREASING <j> ( c ) CONSTANT 0
(e) RATE OF CHANGE OF CURRENT AND FLUX IN POLES
E. SHADED-POLE INDUCTION MOTOR
Fig. 3-5 Different Starting Techniques (ContJ
18
MOTOR CONTROLS
EXPERIMENTS DC CONTROL OFANACMOTOR
CO
H
Z
LU
o
DC
LU
^1/2 LOAD |
y y S ^ V^ RAT ED LOAD
100
1/2 LOAD
^ *** I |-*— RATED VOLTAGE
r I I /
A J H 3/4 RATED VOLTAGE
/- ^ 7^- ■ 1/2 RATED VOLTAGE
/ / ' I
/ / H RATED TORQUE
r/ i
TORQUE
Fig. 3-6 Torque-Slip Characteristics at Different Applied Voltages
degree in single-phase motors, particularly,
and in small induction-type motors, in gen-
eral. However, it is a most unsatisfactory
method of speed control for poly-phase mo-
tors. As seen in figure 3-6, the torque falls
drastically at decreased voltages and speeds.
At reduced voltages the motor may stall be-
fore producing sufficient torque to drive the
load.
This method of speed control, in which
a change of slip is produced by a change in
primary voltage, is feasible for small single-
phase fans or blowers, where the required
CONTROL WINDING
ARMATURE
torque is low at low speeds, and for similar
other loads.
Another type of small AC motor that is
frequently used where speed control is desir-
able is the two-phase induction motor. Figure
3-7 shows the two-phase motor and the phase
relationships. A constant excitation voltage
is applied to one stator winding. A variable
amplitude voltage is applied to the other stator
winding, either 90° leading or 90° lagging the
excitation voltage. The phase relationship
determines the direction of rotation, and the
amplitude of the control voltage determines
the speed of rotation.
EXCITATION CONTROL IN
ONE DIRECTION
CONTROL IN OTHER
DIRECTION
EXCITATION WINDING
Fig. 3-7 The Two-Phase Motor
19
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
MOTOR CONTROLS
Fig. 3-8 Simple AC Motor Control Circuit
A very simple SCR circuit for controNing
an AC motor is shown in figure 3-8. In this
circuit the DC voltage can control the firing
of the SCR from half-cycle conduction to
about one-fourth cycle conduction. This is
usually not a very practical control circuit.
It can be modified as shown in figure 3-9 to
give a different range of control nearer the
full rating of the motor.
This circuit (figure 3-9) will vary the
supply voltage to the motor from a full cycle
to three-quarters of a cycle. By using two
SCRs and two diodes, the motor can be con-
trolled from half power to full power. Figure
3-10 shows a simple DC-controlled SCR cir-
cuit for use with an AC motor.
The circuit in figure 3-9 is an extension
of the one in figure 3-8 and figure 3-10 is an
extension of figure 3-9. You may notice that
the circuit in figure 3-10 is a bridge circuit
used somewhat differently than normal. In
this case the bridge (points A to B) is shorted
and the load is external of the bridge. If the
AC motor in the external circuit were re-
moved, a DC motor or other DC load could
be inserted between points A and B.
Fig. 3-9 Simple SCR Motor Control Circuit
20
MOTOR CONTROLS
EXPERIMENT 3 DC CONTROL OF AN AC MOTOR
Fig. 3-10 Full Wave Motor Control Circuit
MATERIALS
1 Variable DC power supply, 0-40V
1 Small induction motor, preferably
fan motor
2 SCR, type CE106 or equivalent
2 Diodes, type HEP 153 or equivalent
2 Resistors, 470£2, 2W
2 Rheostats, 15012, 15W
1 Oscilloscope
PROCEDURE
1 . Construct a circuit like the one in figure 3-1 1 . Adjust the DC voltage to zero.
CAUTION: Make sure that the common side of the power supply is connected
to the common side of the line.
VARIABLE
DC-PS
Fig. 3- 1 1 SCR Motor Control Circuit
2. Observe and record the waveform across the motor as the speed is varied by adjusting
the DC control voltage.
3. Record data. Then plot a graph of motor speed (co) with respect to firing angle (0-|) and
to the average voltage ( V m ) across the motor.
21
EXPERIMENTS DC CONTROL OF AN AC MOTOR
MOTOR CONTROLS
AC MOTOR
AC SUPPLY
Fig. 3-12 Full Wave Motor Control Circuit
4. Change the circuit as shown in figure 3-12 but omit the rheostat (R).
5. Repeat step two. Then put the rheostat in the gate circuit of the SCR that requires the
smaller firing current (fires first) so that the SCRs can be balanced in firing angle
CO
m
CO
m
j
Fig. 3-13 The Data Tables
22
MOTOR CONTROLS
EXPERIMENTS DC CONTROL OF AN AC MOTOR
6. Adjust the gate rheostat until both SCRs fire at about the same angle as the DC voltage
is varied.
7. Record appropriate data. Then plot a graph of motor speed (co) with respect to firing
angle (d<\ ) and to the average voltage (Vm) across the motor.
ANALYSIS GUIDE. The synchronous motor can be speed-controlled, to some extent, by con-
trolling the average voltage, which in turn changes the slip. Although there are different ways of
controlling the average voltage, one circuit may be more desirable than another. Discuss the dif-
ferences yoif observed and indicate which you think is the best method.
PROBLEMS
1 . Was the amount of slip the same for the same average voltage across the motor while
using the two different circuits? Why?
2. Was the slip the same for the same SCR firing angle in the two circuits? Why?
3. Could the circuit of figure 3-8 be used to reduce the motor speed only a little bit
from the rated value? Why?
4. Which control circuit do you think is best? Why?
5. Using a parts catalog, compute the cost of parts for figures 3-1 1 and 3-12. (Do not
include the motor.) Which circuit is cheaper?
23
experiment /f Ac AND DC VOLTAGES FOR
jT SCR MOTOR CONTROL
INTRODUCTION. An SCR is a very efficient device for controlling AC motors. There are sev-
eral different basic circuits for operating the SCR. One method is to use a combination of AC
and DC voltage. This exercise deals with accomplishing SCR motor control usinq a combination
of AC and DC voltages.
DISCUSSION. Usually, motors with multiple
norsepower ratings require control considera-
tions beyond those of the fractional horse-
power size. This discussion will not include
the control of larger motors.
The principal method of speed control
used for fractional horsepower motors is the
method of primary line voltage control. This
type of control is frequently used with several
types of DC motors and single-phase, induc-
tion-type, shaded-pole, reluctance, series and
universal AC motors.
The basic relationship between torque,
field flux, and armature current in the DC
motor is
(4.1)
where
T = torque
K2 = a proportionality constant
0f = effective field flux
'dc = armat ure current
Since is a function of E dc , the torque
and resultant speed of the DC motor can be
controlled by controlling the line voltage. An
AC motor, however, has so many more vari-
ables that a simple torque equation does not
tell the whole story. A useful equation for the
torque of an AC motor is
T (for any slip) = K t 0 s E £r R r
R r 2 + (sX £r )2
(4.2)
where T = torque
K t = a torque constant for the
number of poles, windings,
units employed, etc.
0 S = the flux produced by each
pole of the rotating mag-
netic field linking the rotor
conductors
E J2r = t ' ie vo| tage induced in the
rotor at standstill
R r = effective resistance at
standstill of all rotor con-
ductors combined
s = ratio of slip speed to syn-
chronous speed
Xj2 r = locked-rotor reactance of
all rotor conductors
combined
This equation is appropriate for large poly-
phase induction asynchronous motors, but
the same factors also affect smaller motors.
Torque in an asynchronous induction motor
depends on slip of the rotor. Figure 4-1 shows
the torque and slip relationship of the AC in-
duction motor at different values of line
voltage.
Although speed control by varying the
line voltage has some disadvantages, it also
has many applications and some advantages.
In applications where speed control by chang-
ing primary voltage is satisfactory, the SCR
control circuit is simple and efficient. The
basic principle of operation is control of the
angle of conduction of the SCR(s). In order
to turn off the SCR, the gate current must be
24
MOTOR CONTROLS
EXPERIMENT 4 AC-DC SCR MOTOR CONTROL
Fig. 4- 1 Torque and Slip Relationship at Different Values of Applied Voltage
below the firing value, and the anode current
must drop below the holding value for a
length of time known as the "turn off" time.
When used in AC circuits, the SCR is turned
off when the anode is reverse-biased during
the negative half cycle. Therefore, the con-
duction angle is controlled by the firing (turn-
on) angle.
The SCR is figure 4-2(a) will be forward-
biased for the first 180° of the waveform
shown in figure 4-2(b). If the SCR were fired
SCR TURN OFF
(B)
Fig. 4-2 SCR Conduction in an AC Circuit
25
EXPERIMENT 4 AC-DC SCR MOTOR CONTROL
MOTOR CONTROLS
as soon after 0° as possible, the SCR would
conduct for nearly 180°, supplying power to
the load during one-half of the applied volt-
age cycle. Now consider that the SCR was
fired at 30°. The SCR will conduct for about
150°, and the average power delivered to the
load will be decreased. In figure 4-2 the SCR
can conduct a maximum of 180° during each
cycle. The average load current, l^c, is a
function of maximum current, l m , and the
firing angle, d<\ :
l m 1 + cos #i
)
(4.3)
For fullwave operation, the equation for aver-
age DC current is
21
'dc-
m
1 + cos d
7T
1
(4.4)
Figure 4-3 shows the output from a fullwave
control circuit with the SCRs being fired at
angle dy. The average current for each half
cycle, as found in equation 4.3, is
'dc
'm 1+cosfl,
7T ( 2 '
(4.3)
Figure 4-4 is a graph of (1 + cos 0^)12
showing how the average current changes
with 0-j.
(A) FULLWAVE RECTIFIER (B) FULLWAVE ALTERNATING
Fig. 4-3 Load Current from Control Circuit
Fig. 4-4 A Graph of (1 + cos 6 j)/2
26
MOTOR CONTROLS
EXPERIMENT 4 AC-DC SCR MOTOR CONTROL
Figures 4-5 and 4-6 are plots of voltage
and power delivered to a resistive load with
respect to the firing angle, 0^ of the SCRs.
Figure 4-5 is for one SCR used in a halfwave
circuit, and figure 4-6 is for two SCRs used in
a fullwave circuit for either DC or AC output.
Fig. 4-6 Full-wave
Phase Control Curve
27
EXPERIMENT 4 AC-DC SCR MOTOR CONTROL
MOTOR CONTROLS
One method of controlling the firing
angle, 0<|, of the SCR circuit is to use a com-
bination of AC and DC voltages. You will
recall that an SCR will fire when the anode is
positive and a critical value of gate current is
reached. According to typical SCR character-
istics, the greater the anode voltage, the less
the critical value of gate current required for
firing. With very careful adjustment of a DC
gate current, the firing angle can be varied
from 0° to 90°. As gate current increases, d*\
decreases. This method of control, however,
is not very stable. A considerable improve-
ment can be made in the control circuit sta-
bility by adding an AC component to the DC
gate bias. The advantage in stability of 6<\ is
shown in figure 4-7. The angle at which the
gate current intercepts the critical firing value
of gate current is shown as a. In figure 4-7 A,
angle a is very small which will result in very
unstable firing. If either the critical value or
the bias changes a small amount, a rather
large change in the firmg angle, 0\, can result.
Figure 4-7B shows the SCR bias as an
AC component on a DC level. This technique
results in a larger angle a which will result in
a more stable firing angle 0-j. The angle 6<\
can be changed nearly 90° (approaching 0°
to 90°) by changing the amplitude of the AC
component or by changing the DC reference
level.
(A) DC GA TE BIAS ( B ) AC AND DC GA TE BIAS
Fig. 4-7 Two Methods of SCR Control
28
MOTOR CONTROLS
EXPERIMENT 4 AC-DC SCR MOTOR CONTROL
MATERIALS
1 115V induction motor
2 SCR Type CE106 or equivalent
1 Transformer, low voltage center
tap secondary
1 Variable DC source, 0-40V
2 Resistance decade boxes (0-1 00k 2W)
1 Variable resistor, 150ft 2W
2 Diodes, HEP 153 or equivalent
2 Diodes, IN319or equivalent
1 Variable AC source
1 Multimeter
1 Oscilloscope
1 Stroboscope
PROCEDURE
1. Construct the circuit shown in figure 4-8. Set the AC voltage to zero volts. Set the DC
voltage source to zero volts. Use decade for R^ = R2 = 25ft.
LINE VOLTAGE
Fig. 4-8 Full wave Motor Control Circuit
2. Increase the DC voltage until the motor just starts to run. Measure the average value of
the gate current (Iq). Observe the motor waveform and using the 150ft variable resistor
in the gate circuit of the SCR that fires first, adjust the resistor until both SCRs fire at
about approximately the same time.
CAUTION: During this experiment, be sure that the gate resistances are large
enough to limit the gate currents to safe values. The gate voltage is the DC plus
the peak AC.
3. Further increase the DC voltage until the motor just reaches full-rated speed. Observe and
record the waveform across the motor. Measure the value of the gate current. Return
the DC voltage to zero.
4. Increase the AC control voltage to 5V RMS and repeat steps 2 and 3.
5. Increase the AC control voltage to 10V RMS and repeat steps 2 and 3.
29
EXPERIMENT 4 A C DC SCR MOTOR CONTROL
MOTOR CONTROLS
6. Increase the AC control voltage to 1 5V RMS and repeat steps 2 and 3.
7. Plot a curve of motor speed with respect to the angle at which the SCRs fire.
DC
5V AC
10V AC
15V AC
Condition
bias
bias
bias
bias
Start
Run
Fig. 4-9 Values of I.
ANALYSIS GUIDE. For motor control from an AC line, the firing of the SCRs with a DC volt-
age was a very critical adjustment. There should be a noticeable difference in the control of the
motor speed when the control voltage has an AC component along with the DC value. Discuss
how your observations tend to verify this conclusion.
PROBLEMS
1. Show graphically the change in DC voltage needed to change the SCR firing angle
through its range with respect to a change in the AC component of the control
voltage.
2. Isthe control gain (sensitivity of control) higher with or without the AC component
in the control voltage? What are the advantages and disadvantages of sensitive
control?
3. Was there a desirable result from having the AC component in the control voltage?
Explain.
4. Is the curve in step 7 linear? Should it be? Why?
30
experiment
5
PHASE SHIFT CONTROL CIRCUITS
INTRODUCTION. Phase shift circuits are frequently employed to produce a vernier shift in
time of a function with respect to a reference alternating signal. This experiment will introduce
some of the phase shift techniques that are used with phase-controlled rectifiers.
DISCUSSION. In order to fire silicon-con-
trolled rectifiers at any angle between 0° and
180°, some type of phase shift circuit is
needed. One of the simplest methods of phase
control for firing an SCR is a basic RC voltage
divider. Figure 5-1 shows an RC circuit with
a step function applied.
In figure 5-1, when Si is closed at t| f
the current at the first instant is limited only
by R so the full voltage is dropped across it
(Vp = E applied). As the capacitor C is
charged, the voltage across C (V c ) increases
and Vr decreases at the same exponential
rate. Vr is the input differentiated, and V c
S 1 CLOSED
TIME
(A) CIRCUIT ■ ( B ) VOLTAGE FUNCTIONS
Fig. 5- 1 RC Circuit and Voltage Functions
K
A
W GATE
Fig. 5-2 RC Diode Firing Circuit
is the input integrated. When the applied
voltage is a sinusoidal waveform, the shape is
not changed since the derivative of a sine
function is a cosine function. The operation
of the circuit in figure 5-1 is more a charge
time delay rather than a phase shift. Figure
5-2 shows the RC diode firing circuit.
During the half cycle when point A is
negative with respect to point B, the diode,
Dj, is forward-biased and allows capacitor C
to charge to the peak value of the applied
voltage. During the other half-cycle, D<| is
reverse-biased and C tries to reverse its charge
through the minimum resistance, R m , and the
phase control resistor, Rp. As long as the
31
EXPERIMENT 5 PHASE SHIFT CONTROL CIRCUITS
MOTOR CONTROLS
Fig. 5-3 Voltage Function in RC Diode Firing Circuit
potential at point A is more positive than that
of point G, the potential of G will rise in the
positive direction. Figure 5-3 shows what
some of the functions might look like.
The point at which V c crosses the zero
reference line can be changed by changing R p .
This point is called 6 and the angle 6 can be
varied from near 0° to 180°. This circuit can
be used to phase-fire an SCR as shown in fig-
ure 5-4. The control circuit in figure 5-4 is
the same as figure 5-3. Diode D2 is used to
protect the gate of the SCR from the negative
voltage during the negative half-cycle. This
circuit will give half-cycle control from no-load
voltage to half-cycle pulses as shown in figure
5-4.
Notice that a second diode cannot be
put across the circuit to give half-cycle to
full-cycle power. If this were done, C1 would
not charge to the negative peak voltage because
the voltage would be dropped across the load.
However, this fact can be used to our advan-
(A) CIRCUIT
(B) LOAD VOLTAGE
Fig. 5-4 RC Diode Circuit for Phase Control of an SCR
32
MOTOR CONTROLS
EXPERIMENT 5 PHASE SHIFT CONTROL CIRCUITS
LOAD
AC
SCR
SEPARATE
FIRING
CIRCUIT
LOAD
VOLTAGE
(A) CONTROL CIRCUIT
(B) LOAD VOLTAGE
Fig. 5-5 Separately-Controlled Firing Circuit
tage in some circuit applications. Figure 5-5
shows a simple half-wave control circuit re-
quiring an outside trigger source.
Figure 5-6 is a slight modification of the
circuit in figure 5-4. The charging diode, D-|/
has a resistor, R2, in series with it giving a
fixed RC charge time constant. Capacitor C
will not charge to the peak value of negative
voltage, but rather to a value depending on
both the time and the magnitude of the neg-
ative voltage.
o
Resistors R m and R p of figure 5-4 are
replaced by R-| and D3 in figure 5-6. In this
circuit, C charges in the negative direction
through R2 and D-|. During the positive half-
cycle, C charges in the positive direction
through R-j and D3. With proper circuit
values, C will charge enough in the negative
direction through R2 and D1 during the neg-
ative half-cycle so that it will not be able to
charge in the positive direction through R-|
and D3 on the positive half-cycle sufficiently
to fire the SCR. Now with the circuits of
o — * 1
. Fig. 5-6 Variation of RC Diode Firing Circuit
33
EXPERIMENT 5 PHASE SHIFT CONTROL CIRCUITS
MOTOR CONTROLS
figure 5-5 and 5-6 combined, the resulting
circuit is known as a fullwave RC diode slav-
ing circuit and is shown in figure 5-7. When
SCR 2 fires, there is no negative voltage to
charge C. Therefore, the more SCR2 con-
ducts, the less C charges in the negative direc-
tion, and the earlier SCRi will be fired by C
charging positive through R^. With the proper
selection of circuit values, SCR-| will fire at
the same angle as SCR2. SCR1 is said to be
slaved or controlled by the firing angle of
SCR2. If the separate firing circuit of SCR2
\s similar to figure 5-4, the result will be a
fullwave control circuit for an AC load.
Another type of phase shift circuit is the
phase shift bridge shown in figure 5-8. As in-
dicated by the phase dots, the voltage at A is
in phase with the anode voltage of the SCR;
the voltage at F is 180° out of phase with the
voltage at point A. The resistor and capacitor,
R and C, are connected from point A to point
F. The SCR will fire when the voltage from
point B to point D goes positive enough to
causes sufficient firing current. Figure 5-8 (B)
shows the voltages at A and F to be equal and
opposite with respect to point B. Since R and
C are in series and the same current must flow
in each, the voltages across them must be 90°
(A) CIRCUIT CONNECTIONS
(B) PHASE RELATIONSHIPS
Fig. 5-8 Phase Shift Bridge Control Circuit
34
MOTOR CONTROLS
EXPERIMENT 5 PHASE SHIFT CONTROL CIRCUITS
out of phase and must add up to the voltage
from point A to point F. As either R or C is
changed in value, the voltage from point B to
point D will remain constant in amplitude
and will shift in phase as shown by the arc.
The firing angle 6 will be zero when R = 0,
and 6 = 180° for R = ~ The geometry of the
diagram in figure 5-8 (B) is such that triangle
BDF is isosceles which means that
0 + (180° -2a) = 180°
1 Z 1 1
6 = 2a= 2 tan" 1 — = 2 tan' 1 coCR
z 2
This circuit would work just as well using
L and R by making Z\ = L and Z2 = R.
MATERIALS
1 Transformer, 1 15V primary, 6.3V to
30V CT secondary
1 Resistor, 1kfi
1 Potentiometer, 10kft linear taper
1 Diode, 1N319
1 Capacitor, 0.1 juF 200V AC
1 Oscilloscope
1 Potentiometer, 25kft linear taper
1 Capacitor, 1.0/xF 200V AC
PROCEDURE
1 . Construct the RC diode phase control circuit shown in figure 5-9-
o-
115V AC
R m =1kn
R p = 10kn
C = 0.1 mF
= 1N442 0R EQUIVALENT
Fig. 5-3 RC Diode Phase Control Circuit
2. With the scope on DC response, set a voltage reference point. "Sync" the scope from the
line voltage so that the trace starts at about 0 voltage on the positive going slope.
CAUTION: The voltage and power in this circuit can be dangerous. Use care
in making measurements and adjustments. Disconnect the power when chang-
ing the circuit.
3. Be sure the common side of the line is connected to common of the circuit and the scope.
Apply the AC voltage to the circuit. Observe the line voltage waveform and set the scope
for a one-cycle display. Take note of the location on the screen of the positive half-cycle
of the line voltage.
35
EXPERIMENT 5 PHASE SHIFT CONTROL CIRCUITS
MOTOR CONTROLS
4. Connect the oscilloscope from point A to common, point B. Record the phase angle (0)
and the slope (dv/dt) at which the voltage crosses the zero line in the positive direction
with respect to the line voltage as R p is varied from one limit to the other. Use about 10
different settings of R p .
5. Plot a graph of peak positive amplitude of the waveform versus the phase angle at which
it crosses zero.
6. Plot the phase angle at which the waveform crosses zero versus the approximate value
of R p .
7. Plot the relative slope of the waveform where it crosses zero versus the phase angle at
which the waveform crosses zero.
8. Construct a circuit similar to figure 5-10.
R = 25kft
C = 1 juF
Fig. 5- 10 Phase Shift Bridge
9. With the scope set up to observe about one cycle of the line voltage and synchronized
with the line voltage, observe the phase shift of the voltage from point A to B.
10. Plot a graph of resistance "R" versus the phase shift of the circuit.
11. Change C to 0.1 yfc and repeat steps 9 and 10,
ANALYSIS GUIDE. A phase control circuit needs to be both stable and easily adjusted. With
respect to the range of phase shift and the ease of obtaining a certain shift, these circuits should
be of particular interest to you. Discuss how each circuit behaved with regard to stability and
ease of adjustment.
36
MOTOR CONTROLS
EXPERIMENT 5 PHASE SHIFT CONTROL CIRCUI TS
dv/dt
R P
(approx.)
R
(approx.)
C= 1.0 /xF
0
C = 0.1 juF
e
v
Fig. 5-11 The Data Tables
PROBLEMS
1 . What are two distinct disadvantages of the circuit in figure 5-9?
2. What might be the result of these disadvantages?
3. What would be an advantage of the circuit in figure 5-9 over the one in figure 5-10?
4. What is the main advantage of the circuit in figure 5-10?
5. What would be the result if R in figure 5-10 were 5kf2 and C were 0.1/xF?
37
experiment
6
PHASE SHIFT CIRCUIT FOR
SCR MOTOR CONTROL
INTRODUCTION. In many applications of fractional horsepower motors, it is desirable to be
able to control the power supplied to the motor from no power to full power. Phase shift cir-
cuits make possible a wide range of phase-controlled firing of SCRs along with providing ease
and stability of a particular setting.
DISCUSSION. Phase shift circuits are used
to control the firing angle of SCRs for a vari-
ety of purposes and in a variety of configura-
tions. Figure 6-1 shows five different basic
circuits in which SCRs could be phase-con-
trolled in single-phase circuits. The waveform
drawing with each circuit shows whether the
circuit would be used for controlling AC loads,
DC loads, or both.
LOAD
1 — 1
AC
SCR
O LOAD 1
a|
I ▼ SCR K RECTIFIER
AC f K ][ D
| CONTROL J 1'
SWITCH
SWITCH
CONTROLLED HALF-WAVE
(a)
CONTROLLED HALF PLUS
FIXED HALF-WAVE
(b)
AC
CONTROLLED FULL-WAVE
(c)
CONTROLLED FULL-WA VE AC OR DC
(d)
O [ LOAD | -
AC
DC SCR
1
mm
CONTROL
CONTROLLED FULL-WA VE FOR AC OR DC
(e)
Fig. 6-1 Basic Forms of Phase Control
38
MOTOR CONTROLS
EXPERIMENTS CIRCUIT/SCR MOTOR CONTROL
6 = 0°
6 = 180°
Fig. 6-2 RC Diode Phase Shift Circuit
The RC diode phase control circuit in
figure 6-2 is compact, inexpensive, and simple,
but can present some problems. The SCR
gate must be protected against the large nega-
tive voltage during the negative half-cycle.
Also, the stability of firing becomes poor at
large angles because the slope of the firing
voltage becomes very flat. The circuit in fig-
ure 6-2 is the basic type shown in figure
6-1 (a).
In this circuit C is not charged to the
peak negative value, as is done in figure 6-2
through D-|, but to a negative value deter-
mined by the firing of SCRs and the charge
path, R2 and D<\ m When point A is negative
with respect to point B, point D will also go
negative until SCR2 fires as shown in figure
6-4. Capacitor C in combination with R2 will
charge according to the time and voltage level
available before SCR2 fires.
Frequently halfwave operation is not
sufficient. The circuit in figure 6-2 can be
used in conjunction with a slaving circuit to
give fullwave control for AC loads similar to
figure 6-1 (c). The slaving circuit is very simi-
lar to the RC diode control circuit as shown
in figure 6-3.
In figure 6-2, C is charged to the peak
value and the charge rate in the positive direc-
tion is varied to control the SCR firing angle.
In figure 6-3, C is charged negative according
to how SCR2 is controlled, and the positive
charge rate is fixed by R-j.
AC
K K
I scr i A
FIRING
CONTROL
CJRCUIT
SCRr
$ = 0°
e = 180°
Fig. 6-3 RC Diode Slaving Circuit
39
EXPERIMENT 6 CI RCUIT/SCR MOTOR CONTROL
MOTOR CONTROLS
Fig. 6-4 Voltage Waveform at Point D in Figure 6-3
When the circuits of figures 6-2 and 6-3
are combined, a fullwave AC control circuit
results as shown in figure 6-5 and is the basic
type shown in figure 6-1 (c).
Another type of circuit for phase control
of SCRs is the phase shaft bridge shown in
figure 6-6. This circuit is also a halfwave con-
trol circuit but has some advantages over the
RC diode circuit. Although the transformer
may be larger and more costly than compo-
nents in the RC diode circuit, the constant
voltage output and more linear control are
distinct advantages. Moreover, the SCR gate
does not have to be protected from extreme
voltages in the negative or positive direction.
Figure 6-7 is the phase shift bridge circuit
modified for fullwave control. This circuit is
of the basic type shown in figure 6-1 (d)
which can be used for either AC or DC loads.
The phase-controlled signal appears between
points A and B. When the anode of SCR-j is
positive, point A will become positive with
respect to point B; and D2 will clamp point B
to the cathode circuit of both SCRs. This
COMMON
AC MOTOR
Fig. 6-5 Fullwave Control Circuit for AC Loads
40
MOTOR CONTROLS
EXPERIMENTS CI RCUIT/SCR MOTOR CONTROL
6 1 *
Fig. 6-6 Phase Shift Bridge Control Circuit
allows the control circuit to fire SCRi with
a positive voltage. When the line voltage re-
verses, the control voltage will also reverse,
and D-| will clamp point A to the SCR ca-
thode circuit and SCR2 can be fired with a
positive voltage.
The circuit of figure 6-7 can be used for
AC loads by connecting the load from point
E to point F with a short from point C to
point D. For DC loads, the short will be
from point E to point F with the load from
point C to point D.
Fig. 6-7 Full wave Phase Shift Bridge Circuit
41
EXPERIMENT 6 CIRCUIT/SCR MOTOR CONTROL
MOTOR CONTROLS
MATERIALS
1 Oscilloscope
1 Multimeter
2 SCRs, CE106 or equivalent
2 Diodes, HEP 153 or equivalent
2 Diodes, 1N319or equivalent
1 Capacitor, 1 //F, 300V AC
2 Resistors, 25ft 2W
1 Decade Box (0-1 00k 2W)
1 Transformer, 1:1 ct. sec.
1 Fan induction motor
1 Motor, 28V DC
1 Variable AC source, 0-50V
1 Stroboscope
PROCEDURE
1 . Construct the circuit shown in figure 6-8. Use the decade box for R p .
Fig. 6-8 RC Diode Phase Shift Circuit
2. Observe the waveform across the load as R p is varied from maximum to minimum. Re-
Fig. 6-9 Full wave Control Circuit for AC Loads
42
MOTOR CONTROLS
EXPERIMENT 6 CIRCUIT/SCR MOTOR CONTROL
4. Adjust R-| and R2, if necessary, to get both SCRs firing at about the same phase angle as
Rp is varied through its range.
5. Record data and plot motor speed and the firing angle of the two SCRs versus the value
of R p .
6. Construct the circuit shown in figure 6-10.
Or
Fig. 6-10 Phase Shift Bridge Control Circuit
7. Record appropriate data and plot motor speed and phase angle of firing versus the value
of R p .
8. Construct the circuit shown in figure 6-1 1 .
Fig. 6- 1 1 Full Wave Phase Shift Bridge Circuit
43
EXPERIMENT 6 CIRCUIT/SCR MOTOR CONTROL
MOTOR CONTROLS
9. Take appropriate data and plot motor speed and firing angle of both SCRs versus R p as
it is varied through its range.
CO
(RPM)
(RPM)
Data from: Fig. 6-8
Fig. 6-9
CO
(RPM)
CO
(RPM)
Fig. 6-10 Fig. 6-11
Fig. 6-12 The Data Tables
44
MOTOR CONTROLS
EXPERIMENTS CIRCUIT/SCR MOTOR CONTROL
ANALYSIS GUIDE. Phase firing silicon-controlled rectifiers is an efficient means of power con-
trol. An important aspect of this type of power control is the circuit used to fire the SCRs.
Discuss each of the circuits used in the experiment with regard to ease and stability of speed
control. Which circuits seemed to work best? Why?
PROBLEMS
1. What are the advantages of the phase shift control circuit over the DC or AC-DC
method of SCR firing?
2. What advantage does the phase shift bridge have over the RC diode phase control
circuit? What disadvantages?
3. Can the RC diode circuit be used for fullwave control? Why or why not?
4. Using a catalog, determine the price of the motor controls used in this experiment.
How do the two circuits compare?
45
experiment THE UNIJUNCTION TRANSISTOR
INTRODUCTION. The unijunction transistor (UJT) is a three-terminal device exhibiting a stable
incremental negative resistance region. This makes possible the design of switching and pulsing
circuits using fewer components than comparable transistor circuits. Iri this exercise we will
investigate some of the characteristics of the UJT.
DISCUSSION. The UJT is a device that can
be operated in a number of different circuit
configurations such that any of the three ter-
minals can serve as signal input or output.
A cross-sectional diagram of the physical
construction of a typical UJT is shown in
figure 7-1. A ceramic disk having the same
expansion coefficient as silicon is used as a
mounting base. A gold film is deposited on
both sides of a slit 10 mils wide in the center
of the ceramic disk. An N-type silicon bar
(single crystal) is laid across the slit and an
ohmic contact is formed between the gold
film and the silicon bar at each end. These
contacts are the base-one (B1) and base- two
(B2) contacts. The resistivity of the silicon
bar is about 120 ohm-cm and the dimensions
are about 8 X 10 X 35 mils. A single P-type
emitter junction is formed by a small alumi-
num wire on the side opposite the base con-
tacts. This emitter junction is usually located
closer to the base-two contact so the device
is not symmetrical.
The operation of the UJT is dependent
upon modulation of the conductivity between
the emitter junction and the base-one contact.
The conductivity of the silicon in this region
is given by
ff = q(MpP + M n n )
(7.1)
N-TYPE
SILICON
BAR
EMITTER (E) I
BASE TWO (B2)
PN JUNCTION
GOLD FILM
CERAMIC BASE
OHMIC BASE
CONTACTS
BASE ONE (B1)
Fig. 7-1 Circuit and Construction of a Bar-Type Unijunction Transistor
46
MOTOR CONTROLS
EXPERIMENT 7 THE UNIJUNCTION TRANSISTOR
where a = Conductivity of silicon
(ohm -1 cm" 1 )
q = Electric charge 1.60 X 10" 19
coulombs
ptp = Mobility of holes in silicon
«250cm 2 volt _1 sec" 1
P = Hole concentration (hole/cm^)
M n = Mobility of electrons in
silicon
« 1200 cm 2 volt" 1 sec" 1
n = Electron concentration
(electrons/cm^)
The term /z p on the right of equation 7.1 can
be neglected when there is an absence of in-
jected carriers from the emitter since the bar
is of N-type material and the holes are the
minority carriers. When the emitter is biased
in the forward direction, holes are injected
into the bar and swept into the emitter-base
region greatly increasing the conductivity.
This increase in conductivity is due to the
increase of holes in the region where they are
injected, causing an increase in the number
of electrons to maintain the space charge
neutrality.
Although the UJT can be used in a vari-
ety of circuit configurations, normal operation
is to ground base one and to bias base two
with a positive voltage, Vbb- Figure 7-2a
shows the normal connection and the sym-
bol for a UJT.
The positive voltage at B2 establishes a
current and an electric field within the silicon
bar and produces a voltage on the N side of
the P-N emitter junction. This voltage is a
fraction, 17, of the applied voltage or voltage
from B1 to B2. The fraction 17 is termed the
intrinsic standoff ratio and is determined by
the spacing of the emitter junction between
the two base contacts. The fraction of the
distance from B2 to B1, that the emitter is
spaced from B1, is equal to the fraction of
Vbb that w "' be present at the emitter junc-
tion. This fraction is called the intrinsic
stand-off ratio 17. If the applied emitter volt-
age, Vg, is less than tjVbb* the emitter junc-
tion will be reverse-biased and only a small
reverse leakage current, 1^0/ w '" f' ow ' n the
emitter. Where the applied emitter voltage
exceeds t?Vbb by an amount equal to the
PN JUNCTION
VOLTAGE
DISTRIBUTION
VOLTAGE AT
PN JUNCTION
(A) SYMBOL FOR THE UNIJUNCTION
TRANSISTOR
(B) PHYSICAL EQUIVALENT
Fig. 7-2 The Unijunction Transistor
47
EXPERIMENT 7 THE UNIJUNCTION TRANSISTOR
MOTOR CONTROLS
forward voltage drop of the emitter diode,
V[), holes will be injected into the bar. Be-
cause of the electric field in the bar, the holes
will move toward B1 and increase the con-
ductivity of the bar in the region between E
and B1. As the emitter current, lg, is in-
creased, the emitter voltage will decrease be-
cause of the increased conductivity so that a
negative resistance characteristic is observed
between the emitter and the base-one ter-
minals.
Figure 7-3 is a typical emitter character-
istic curve for a UJT. Two important points
on this curve are the peak point and the valley
point. The slope of the curve is zero at these
two points. The emitter diode is reverse-
biased to the left of the peak point and this
region is known as the cut-off region. Be-
tween the peak point and the valley point is
the negative resistance region. The saturation
region is to the right of the valley point. In
the saturation region, conduction between E
and B1 is limited by the surface and bulk
recombination of the holes and electrons.
The peak point depends on Vgg and the in-
trinsic stand-off ratio. The valley point de-
pends on the resistance in series with B1 and
B2. The valley voltage, V v , decreases as the
resistance in series with B2 is increased, and
rises as the resistance in series with B 1
increases.
■PEAK POINT
EMITTER
VOCTAGE
Fig. 7-3 UJT Emitter Characteristic Curve
48
MOTOR CONTROLS
EXPERIMENT 7 THE UNIJUNCTION TRANSISTOR
MATERIALS
1 Unijunction transistor, 2N21 60
or equivalent
1 Resistor, 1kfi 1/2W
PROCEDURE
1 . Construct the circuit shown in figure 7-4.
E UJT
0 j 1 1 1
Fig. 7-4 UJT Test Circuit
2. With Vfjfj = 0 and Vee negative with respect to B1, take data and plot Ie versus Ve as
Vee is varied from zero to about ten volts. Take enough readings to plot a smooth curve.
3. With VgB = 0 and Vee positive with respect to B1, take data and plot Ve versus Ie as
Vee ' s increased. The emitter current should not exceed about 30 mA.
4. Set Vbb = + 5 volts; then take data and plot Ie versus Ve and I B2 as ^EE ' s increased in
the positive direction. Plot enough points to show the peak, negative slope and valley
of the emitter voltage curve.
5. Set Vbb = +10 volts and repeat the plots as in step 4.
6. Set Vbb = + 20 volts and repeat the plots as in step 4.
7. Show V p , l p , V v , and l v on all the emitter voltage curves.
ANALYSIS GUIDE. The unijunction transistor has unique characteristics that make it quite
useful in control circuits. Being familiar with these basic characteristics is a big asset in under-
standing control circuit techniques. Discuss the extent to which your results agreed with the
points presented in the discussion.
2 Variable DC power supplies, 0-40V
1 Multimeter
49
EXPERIMENT 7 THE UNIJUNCTION TRANSISTOR
MOTOR CONTROLS
V BB
= 0,
V BB
= o,
V BB = 5V,
V BB = 10V,
V BB = 20V,
V EE
= -E
v EE
= +E
V EE = +E
V EE = +E
v
EE = +E
V E
'e
v e
'e
v E
'E
'b2
V E
'e
'b2
V E
'e
'b2
Fig. 7-5 The Data Table
PROBLEMS
1 . Calculate the intrinsic standoff ratio for each of the values of Vbb-
2. How much does the resistance from Bi to B2 change when the UJT goes into con-
duction?
3. How much does the resistance from the emitter to base one change when the UJT
goes into conduction?
4. Can base two be used as a control or signal input? Explain.
50
experimen t
8
UNIJUNCTION TRANSISTOR
FOR SCR CONTROL
INTRODUCTION. The characteristics of the unijunction transistor (UJT) make it particularly
useful in circuits where pulses are involved. The UJT is frequently used in control circuits to
fire silicon-controlled rectifiers. In this exercise we will examine some of the applications of the
UJT in SCR control circuits.
DISCUSSION. The characteristics of the uni-
junction transistor make it a very useful device
in many electronic applications. One of these
applications is to supply the gate-firing cur-
rent for silicon-controlled rectifiers.
When SCRs are used for power control
from an AC source such as in motor controls,
battery chargers, or arc welders, the SCR is
fired at some variable phase angle with respect
to the source voltage. For maximum power
delivered to the load, the SCR is fired when
the anode voltage just begins to go positive,
and it conducts for the full 180°. If less than
full power is to be delivered to the load, the
SCR is fired at some phase angle after the
anode goes positive. Therefore, a controllable
circuit is needed that can provide a positive
current sufficient to fire the SCR. This cur-
rent must be controllable from about 0° to
180°. Figure 8-1 illustrates the firing angle
with respect to the SCR anode voltage.
Such circuits as the RC diode phase con-
trol circuit and the RC phase shift bridge will
produce a voltage that goes positive at a vari-
able angle between 0° and 180°. These cir-
cuits are, however, affected by loading when
they are connected to the SCR gate and also
by supply voltage changes. The circuit could
be built to compensate for loading and voltage
regulation, but more power would be con-
sumed in the control circuit, components
would be larger, and it would be difficult to
take into account the differences in SCRs and
the effect of temperature changes. The uni-
junction transistor (UJT) can be used as an
isolation device between the phase shift cir-
cuit and the SCR.
ANODE
VOLTAGE
0° $ 180°
Fig. 8-1 SCR Phase Control
51
EXPERIMENT 8 TRA NSISTOR/SCR CONTROL
MOTOR CONTROLS
B2
D 1
'BB
IN
A
SILICON |
BAR
B1
B1
CERAMIC
DISC
IS
(A) EQUIVALENT CIRCUIT
(B) PHYSICAL CONSTRUCTION
Fig. 8-2 Unijunction Transistor
One of the simplified equivalent circuits
for the UJT is shown in figure 8-2(A). With no
emitter current flowing, there is a relatively
high resistance between base one and base
two. This resistance is the result of two resis-
tive connections, base one and base two, at
opposite ends of an IM-type silicon bar. The
emitter connection or junction is between the
base connections on the opposite side of the
bar and is usually closer to the base-two con-
nection. The resistance of the bar is divided
into two parts, Rg2 from base two to the
emitter junction, and Rgi from base one to
the emitter as shown in figure 8-2. Since the
silicon bar acts as a resistive element, its
voltage drop will be proportional to the resis-
tance from base one to base two. The poten-
tial at the emitter junction with respect to the
base-to-base voltage will be in the same ratio
as is the distance from the emitter to base
one to the length of the bar. The portion of
the base-one-to-base-two voltage that appears
at the emitter junction is called the intrinsic
standoff ratio (17). Since the emitter connec-
tion is a P-N junction, it will not conduct so
long as the voltage applied to the emitter is
less than that of the bar at the junction. With
this junction reverse-biased, the emitter-to-
base-one circuit is nearly an open circuit and
the base-two-to-base-one circuit is a relatively
high resistance. These characteristics allow
the UJT to isolate two circuits, especially be-
tween emitter to base one. When the emitter
voltage causes the emitter junction to be
forward-biased, holes are injected into the
silicon bar and the result is that conductivity
is greatly increased between the emitter and
base one. This is illustrated by a reduction in
Rgi, figure 8-2(A). The new value of Rg<|, or
the saturation resistance, changes from several
thousand ohms to as low as 30 to 50 ohms
very quickly. This will also significantly re-
duce the resistance from base one to base two.
This sudden conduction characteristic makes
the UJT an ideal device for supplying the fir-
ing current to an SCR.
52
MOTOR CONTROLS
EXPERIMENTS TRANSISTOR/SCR CONTROL
'B1
Fig. 8-3 Basic UJT Relaxation Oscillator
A typical UJT circuit for SCR control is
shown in figure 8-3. The basic UJT trigger
circuit used in SCR control circuits is a simple
relaxation oscillator. In this circuit, the capac-
itor, Ci, is charged through Ri until the
emitter voltage reaches the voltage where the
emitter junction is forward-biased. At this
time the UJT turns on and discharges Ci
through Rg<| and Rg2- When the emitter
voltage reaches a value of about 2 volts with
respect to base one, and the emitter current
drops below the valley current, the emitter
ceases to conduct. The UJT turns off and
the cycle is repeated. The period of oscilla-
tion, T, is fairly independent of the supply
voltage and temperature. The period T can
be found by
t=7 =r i c i
Kn T^ = Z3R i c i log ioTT^
where r\ = intrinsic standoff ratio.
The design limits of the UJT relaxation
oscillator are very broad. Usually Rg-j is
limited to a value below 100 ohms but it may
be as high as 2k or 3k ohms in some cases.
Resistor R<| will probably be between 3k
ohms and 3 megohms. The lower limit is set
by the requirement that the load line formed
by Ri and Vi intersect the emitter curve to
the left of the valley point or the UJT will not
turn off (see figure 8-4). In other words, the
voltage divider formed by Ri and the satura-
tion resistance of the UJT must be a ratio
such that is below the cut-off value of
about 2 volts. The upper limit for R<| is set
by the requirement that the current flowing
into the emitter at the peak point must be
1Gt— ^V-, = 15V
IF Ri IS SMALLER THAN
THIS THE UJT WILL NOT
TURN OFF.
I F (mA)
Fig. 8-4 Limiting Condition for Rj
53
EXPERIMENT 8 TRANSISTOR/SCR CONTROL
MOTOR CONTROLS
Fig. 8-5 Unijunction Circuit for SCR Control
greater than l p for the UJT to turn on. The
range of supply voltage, Vi, is usually from
about 10 to 35 volts. The low value is deter-
mined by the required signal levels and the
high value is determined by the allowable peak
power dissipation of the UJT.
As mentioned above, the UJT relaxation
oscillator is relatively insensitive to the supply
voltage value. This is because the UJT con-
ducts when the emitter reaches a certain per-
cent of the base-one-to-base-two voltage which
is about the same as the supply voltage. With
a particular RC time constant, C will charge
to a certain percent of the applied voltage in
the same time, regardless of the value of the
supply voltage. Since the UJT relaxation os-
cillator has this characteristic of being insensi-
tive to supply voltage, it can be supplied from
a rectified and unfiltered source. This allows
simplification of the circuit and provides a
means of synchronizing it as a phase control
circuit. Figure 8-5 shows the UJT relaxation
oscillator operating from a rectified and un-
filtered voltage and used to fire a silicon-
controlled rectifier.
The voltage between points A and B is a
full-wave rectified positive pulse and the RC
time constant is such that the UJT will con-
duct before the end of each half-cycle. In
fact, Ri can be changed such that the UJT
will conduct from very early in the half-cycle
to near the end of the half-cycle. If the in-
trinsic standoff ratio of the UJT is about 0.63,
which is a reasonable figure, then the UJT will
conduct after about one time constant. With
R-I = 10 kfl and C = 1.0 /iF f the time con-
stant (r) equals 0.01 seconds. The half-cycle
pulse is 1/120 of a second so a time constant
of 1/100 second is long enough to give a good
range of control.
The resistor R-| is returned to base two
rather than the source voltage to help prevent
too much emitter current when the emitter
base-one resistance is low. With no resistor in
series with R 1 , and when R 1 is at a minimum,
a large current would flow from the soured
through the base along with the discharge cur-
rent from C. Figure 8-6 shows waveforms of
the circuit in figure 8-5 and shows Vb2 drop-
ping, which would help to reduce the peak
base current. Also, a resistance could be
added in series with R<| as a minimum resis-
54
MOTOR CONTROLS
EXPERIMENTS TRANSISTOR/SCR CONTROL
Fig. 8-6 Waveforms for the Circuit in Figure 8-4
tance to protect the base junction. A firing
angle of zero cannot be attained because the
capacitor, C, has to charge enough to supply
the energy needed to fire the SCR.
This circuit can be made a little more
linear by adding a zener diode regulator. This
also permits using a larger voltage and achiev-
ing a firing angle closer to zero. Figure 8-7
shows the waveform change when using the
zener-regulated voltage source.
It can be seen in figure 8-7 that by using
the zener regulator, a larger voltage source can
be used and then regulated down to a safe
value for the UJT circuit. Since the larger
applied voltage is regulated to certain values,
the voltages increase to this value more quick-
ly than does the unregulated voltage. This
makes possible the charging of C to the UJT
firing voltage sooner, which, in turn, fires the
SCR so that the firing angle can be adjusted
closer to zero degrees.
MAX. SAFE VOLTAGE
FOR UJT
CHARGE NEEDED ON C
TO FIRE; SCR
MINIMUM FIRING ANGLE (REGULATED)
Fig. 8-7 Comparison of Regulated and Unregulated Supply Voltages
55
EXPERIMENTS TRANSISTOR/SCR CONTROL
MOTOR CONTROLS
MATERIALS
1 Multimeter
1 Oscilloscope
1 Variable AC voltage source, 0-50V
1 Transformer, 1:1 center tap secondary
1 Unijunction transistor, 2N2160
or equivalent
1 Potentiometer, 10k-ohm 1W
1 Capacitor, 1.0 mF 600W, VDC
PROCEDURE
1 . Build a circuit as shown in figure 8-8
J K
1 Transformer, 1:4
2 Resistors, 5k-ohm
4 Diodes, 1N319 or equivalent
2 SCR type CE106 or equivalent
1 AC fan motor, 110V AC
1 Resistor, 1k-ohm 5W
1 Zener diode, 20V
1 Stroboscope
AC
SOURCE
VARIABLE
AC
SOURCE
Fig. 8-8 Unijunction Control Circuit
2. Sync the scope on the line voltage so that the sweep starts near zero volts on the positive
going slope. Note where the zero point is on the negative going slope. Observe the wave-
form across capacitor C. You should be able to vary the negative going slope from near
zero degrees to near 180°.
3. Construct the circuit in figure 8-9 and connect it to the secondary of T2 in figure 8-8.
T., •
47n
r-WV
SCRi
47^
115V AC
SCR 2 U 2
A
i
FAN MOTOR
Fig. 8-9 Circuit for Phase Control of an AC Load
56
MOTOR CONTROLS
EXPERIMENTS TRANSISTOR/SCR CONTROL
I 1 * 1 T 2
Fig. 8-10 Modified Control Circuit
4. Take data and plot motor speed (co) versus the AC voltage across the motor (V m )
(measured with the multimeter).
5. Measure and plot the value of R3 (figure 8-8) versus the AC voltage across the motor.
6. Record and plot the firing angle (0) of the SCRs versus the AC voltage across the motor.
7. Modify the control circuit of figure 8-8 as shown in figure 8-10. {Add Rr, D3 and in-
crease the voltage).
8. Record and plot the SCR firing angle versus the value of R3.
9. Modify the circuit in figure 8-9 as shown in figure 8-1 1 .
10. Observe and compare the waveforms across the motor and compare the control of the
motor with that of the previous circuit (step 9).
SCR*
PRIMARY
D 2 T 2 SECONDARY Q
115V AC
MOTOR
<=>
Fig. 8- 1 1 Circuit for SCR Control
57
EXPERIMENTS TRANSISTOR/SCR CONTROL
I
MOTOR CONTROLS
CO
R 3
e
Results, Figure 8-8 Results, Figure 8-10
Fig. 8-12 The Data Tables
ANALYSIS GUIDE. The unijunction transistor makes possible a rather simple but effective
circuit with low power consumption for phase firing of silicon controlled rectifiers. This type
of circuit is easily adaptable to a variety of SCR circuit configurations. Discuss the results you
got in the experiment and compare them to other types of SCR control.
PROBLEMS
1 . What might be the two main advantages of using a unijunction transistor in an SCR
control circuit? Explain.
2. In what way does the pulse control give a distinct advantage over an ordinary RC
diode phase shift circuit?
3. How would you modify this circuit to respond to a small control voltage or current?
Explain.
4. Using catalogs determine the price of the UJT circuit in this experiment.
58
experiment
9
MOTOR CONTROL BY PULSE
HEIGHT VARIATION
INTRODUCTION. There are many ways of conveying control information. One method is to
amplitude modulate pulses. In this exercise we will investigate the use of changing the height of
pulses as a means of controlling a DC motor.
DISCUSSION. One of the many means of
sending information from one place to another
is by means of a string of pulses that vary in
height with respect to some function. The
height of the pulse may be proportional to a
voltage, temperature, speed, flow rate, or any
one of many other functions depending on
the transducer and modulation arrangements.
A more complex function can also be con-
veyed by a pulse train modulated in ampli-
tude.
Figure 9-1 shows a sinusoidal waveform
represented by the variation in height of a
series of pulses. Notice that the repetition
rate of the pulses must be considerably higher
than the frequency of the sinewave signal. In
the case of a complex waveform, the repeti-
tion rate of the pulses must be somewhat
higher than the highest frequency component
of the complex signal; otherwise, some of the
information may be lost.
The process of demodulation, or obtain-
ing the original information from a train of
pulses, may involve complicated circuitry or
it may be very simple. If the pulses are sym-
metrical (spacing and width the same) and are
of constant amplitude, the average voltage is
one-half the voltage of an individual pulse.
The pulses in figure 9-2 are 10 volts in ampli-
tude, and the average voltage is five volts.
These pulses applied to a DC motor would
provide the same power as would a five-volt
DC source (neglecting inductive effects). If it
were desirable to operate a 28-volt DC motor
at full power from a train of such symmetrical
pulses, the pulses would need to be 56 volts
in amplitude.
PEAK VALUE
Fig. 9- 1 Sinewave Modulation of Pulse Amplitude
10V
T
5V
AVERAGE
VOLTAGE
Fig. 9-2 Average Voltage of a Pulse Train
59
EXPERIMENT 9 MOTOR CONTROL/PULSE VARIATION
MOTOR CONTROLS
LOW LEVEL SIGNAL
AVERAGE
DC LEVEL
pulse nln n power
AMP LOAD
amplifier n n n shaper nln
ruukiui ti n nil II II MnnnlHtni — i
'H l_l
Fig. 9-3 DC Control by Variation in Pulse Amplitude
Among the advantages in handling in-
formation in the form of variations in pulse
amplitude are those relating to amplification.
The train of pulses can be considered as an
AC signal that is clamped to zero. There are
some problems in achieving stable gain, multi-
ple stage coupling, and proper zero reference
with DC amplifiers. This is usually accom-
plished by some process of chopping, or some
other method of periodic referencing and
feedback. Information being conveyed by the
amplitude of pulses in a train is the same as
the chopped DC, but can also be handled as a
clamped AC by more conventional AC-coupled
amplifier stages. Stability and multistage
coupling are not so much of a problem with
AC amplifiers.
The pulses in the pulse train do not need
to be symmetrical in order to convey the de-
sired information. Figure 9-3 shows a block
diagram of a possible technique for DC motor
control using relatively narrow pulses that
vary in amplitude.
Low level signals can be transmitted
easily and amplified to the desired level. Then
the pulses can be shaped or extended to the
desired width and used to drive the power am-
plifier or device that operates the load. In
this type of circuit, the power amplifier is
acting as an impedance in series with the
motor and power supply.
In the circuit of figure 9-4, the voltage
of the power source is equal to the average
value of the pulses times the voltage gain of
the power amplifier. If the voltage gain of the
power amplifier is varied from zero to one,
the average voltage across the motor will vary
from zero to the average amplitude of the
pulse train. With the power amplifier voltage
PULSES in
E p = AVERAGE VOLTAGE
OF PULSES
A pa = VOLTAGE GAIN OF
POWER AMP
Fig. 9-4 Motor Control by Pulse Amplitude Variation
60
MOTOR CONTROLS
EXPERIMENT 9 MOTOR CONTROL/PULSE VARIATION
PULSES
i — i
pa
DC
SUPPLY
ID
MOTOR
(A) EQUIVALENT CIRCUIT
PULSES IN
E
DC
SUPPLY
W CONTROL CIRCUIT
Fig. 9-5 An Equivalent Circuit and Simple Control Circuit
gain held constant at one, for example, the
average voltage across the motor, and the
speed of the motor, can be controlled by the
amplitude of the pulse train.
Since the power amplifier furnishes the
power to operate the motor, it must operate
from a power source. Figure 9-5 (A) shows a
simple circuit analogy for this type of motor
control and figure 9-5 (B) shows the actual
circuit.
An important consideration in this cir-
cuit is the power rating of the amplifier. The
maximum power dissipation in the amplifier
will occur when the input impedance of the
motor and the output impedance of the am-
plifier are equal. When this is the case, the
voltage of the source is equally divided be-
tween the two, and the motor and the ampli-
fier dissipate the same amount of power. But,
notice that the motor is not operating at one-
half its rated power. If the total resistance in
the circuit is doubled, the current is reduced
to one-half the original value, and the voltage
is equally divided. The power dissipated by
61
EXPERIMENT 9 MOTOR CONTROL/PULSE VARIATION
MOTOR CONTROLS
the motor would then be one-half its normal
current times one-half its normal voltage, or
one-fourth its normal power. The minimum
power rating of the amplifier would then be
one-fourth of the maximum power to be de-
livered to the motor. If the motor in figure
9-5 offers a constant three-ohm impedance,
and the power source is 12 volts, then the
voltage, current, and power functions will vary
as shown in figure 9-6. Since the amplifier
basically serves as a resistor, the power curve
for the resistor is also the power curve for the
amplifier:
E|y| = voltage across the motor
Ep = voltage across the series resistor
I = current supplied by the power
source
P|YI = power delivered to the motor
Pp = power dissipated in the resistor
VALUE OF H IN OHMS
Fig. 9-6 Voltage, Current, and Power Curves
MATERIALS
1 DC motor, 28 volts 1 Resistor, 47£2, 2W
1 Pulse generator (1kHz - pulse 1 DC P ower supply, 0-40 volts
width 250 to 900 ms) ! Multimeter
1 NPN Transistor - Power, 2N3055 1 Stroboscope
or equivalent.
62
MOTOR CONTROLS
EXPERIMENTS MOTOR CONTROL/PULSE VARIATION
PROCEDURE
1 . Construct the circuit shown in figure 9-7.
PULSE
GENERATOR
Fig. 9-7 The Experimental Circuit
T = 250 jus
T = 500 jus
T = 750 ms
T = 900 jus
V P
CO
V P
CO
CO
v p
CO
Fig. 9-8 The Data Table
63
EXPERIMENT 9 MOTOR CONTROL/PULSE VARIATION
MOTOR CONTROLS
2. Set the pulse generator for 1kHz pulses, 500 microseconds in width.
3. Increase the pulse amplitude (Vp) in two-volt steps from zero to 30 volts and record the
average motor voltage (V m ) (as read by a voltmeter) for each pulse amplitude and the
motor speed (co).
4. Change the pulse width to 250 microseconds and repeat step 3.
5. Change the pulse width to 750 microseconds and repeat step 3.
6. Change the pulse width to 900 microseconds and repeat step 3.
7. Plot a graph of the power dissipated in the motor (V m 2 may be used since it is propor-
tional to P m > and in the transistor versus the pulse amplitude as in step 3 with the pulse
width at 900 microseconds.
8. Plot a curve of motor speed versus pulse height for each pulse width.
ANALYSIS GUIDE. In analysis of the data taken during this experiment, you should take par-
ticular note of the range of motor voltage with respect to pulse width as pulse amplitude is
varied. Also, the power dissipated in both the amplifier and the motor should be noted. Discuss
the relationship between motor speed and pulse amplitude for various pulse widths.
PROBLEMS
1. Plot a graph of the motor voltage and computed average voltage for each width of
pulse versus the pulse amplitude.
2. What effect does the pulse width have on the range of motor control? Why?
3. If motor control information is represented by the amplitude of narrow pulses,
why is a pulse shaper needed to make the pulses wider?
64
experiment // / DIGITAL MOTOR CONTROL
INTRODUCTION. Pulse width variation and pulse frequency variation are two ways of using
constant amplitude pulses to convey information. The speed of DC motors can be easily and
efficiently controlled by these two types of digital information.
DISCUSSIOlM. Many applications of motors
require a wide range of speed variations or
control. A series DC motor is particularly use-
ful where variable speed and high starting
torque are required. One of the simplest
methods used to control the speed of a motor
is to control the input power to the motor.
The most common method of speed con-
trol of small motors is to use a variable resistor
in series with the motor. The power is divided
between the resistor and the motor with the
result of power loss in the resistor. This
causes poor efficiency and requires a resistor
with a high power rating. Another disadvan-
tage is poor speed regulation when the motor
load changes.
A digital or switching mode of control is
much more efficient and has much better
speed regulation as motor load is changed. It
allows more power handling capability with
fewer and smaller components.
A digital system deals with discrete values
or numbers in contrast to the analog system
which deals with values on a continuously
variable scale. A simple example is a rheostat
and a resistance decade box. The resistance
of a decade box can be varied only in discrete
amounts that can be precisely read and dupli-
cated. The rheostat can be changed in resis-
tance along a continuous range, but the value
at any point can be read only as accurately as
the dial can be calibrated and interpreted.
The duplication of a specific setting is also
subject to the accuracy of reading and posi-
tioning of the dial. In analog computers the
information is represented by changing levels.
Amplifier gain and drift become very critical
because they affect the accuracy of the com-
puter output. Accumulative error from several
stages is a serious problem in analog com-
putors.
The digital computer usually processes
information in the binary (base two) number
system. Since there are only two values in
the base two system, zero and one, the cir-
cuits will usually operate in one of two stable
conditions: saturation or cutoff. Informa-
tion is converted to a binary number with the
number of digits depending on the accuracy
needed. All of the computer processes are
carried out as arithmetic functions with the
only error being rounding of numbers or com-
plete circuit failure. Errors due to circuit
gain, drift or other instabilities are almost
nonexistent.
Switching mode motor control has some
distinct advantages even though it may not be
completely digital. As previously mentioned,
motor control is usually accomplished by
varying the input voltage or power. As an
example, a 28-volt motor that draws 2 amps
under full load is to be operated at one-fourth
full power or 14 watts input power. If a
series resistor or other device is used as shown
65
EXPERIMENT 10 DIGITAL MOTOR CONTROL
MOTOR CONTROLS
(C) 1/4 POWER
Fig. 10-1 Motor Control By Using Series Resistance and Switching
in figure 10-1 (B), it will consume as much
power as the motor, assuming the motor im-
pedance remains relatively constant. To oper-
ate the motor at one-fourth power in the
switching mode technique shown in figure
10-KC), full power is applied to the motor
for one-fourth of the time, and switched off
for three-fourths of the time. The average
power supplied to the motor is, then, one-
fourth of the full power. While in the off
condition, ideally no current flows so no
power is lost in the control device. If the
control device drops about one vojt while in
the on condition, which is typical of most
semiconductor devices, it will consume power
at the rate of two watts, but it is on only one-
fourth of the time. Therefore, the average
power consumed in the control circuit is about
one-half watt, rather than 14 watts by the
other method.
66
MOTOR CONTROLS
EXPERIMENT 10 DIGITAL MOTOR CONTROL
TRANSISTOR
SWITCH
I
SPEED
CONTROL
0
POWER ON
POWER OFF
MAXIMUM
SPEED
TIME
HIGH
SPEED
MEDIUM
SPEED
SLOW
SPEED
1/100 SEC
Fig. 10-2 Motor Control by Pulse Width Variation
There are two basic ways of changing
the average power delivered to a load by
switching. One method is to vary the ratio of
on— off time with constant frequency or rate
of switching. The other is to have a constant
on or off time and vary the frequency or rate
of switching. These techniques might be con-
sidered as pulse width and frequency varia-
tion, respectively. Figure 10-2 is a block dia-
gram of a switching mode motor control
circuit and some typical waveforms showing
pulse width variation to control the average
power to the motor.
If the pulse width or spacing were a
digital function, the control system would be
completely digital. With the pulse width or
spacing being an analog function, the control
circuit could be considered switching mode
control with analog variation.
The circuitry needed for controlling a
small motor with a pulse train that is varied,
either in frequency or width, is very simple.
67
EXPERIMENT 10 DIGITAL MOTOR CONTROL
MOTOR CONTROLS
28V
28V
DC MOTOR
30V
Fig. 10-3 Simple Switching Mode Control Circuit
Figure 10-3 shows a power transistor being
gated by a pulse input, 28 volts in amplitude.
The motor is in the emitter circuit of a com-
mon collector transistor circuit. This circuit
has a voltage and current gain approximately
equal to a and 0, respectively, of the transis-
tor. The value of a is just a little less than
one (.9 to .98, for example) and j3 might be
over a hundred for small power units to as
low as ten for high power units. For large
amounts of power, switching units can be
parallel and a driver amplifier will probably
be needed to operate the power switch.
Figure 10-4 shows a comparison of pulse
width and frequency variation required to
change the average voltage delivered to the
motor.
Since total power is the integral of the
area under the power curve (pulses), the aver-
age power is the average area over a period of
time. The pulses are considered rectangular
and of constant height, and the average volt-
age is the percent of time "on". Knowing the
average time "on" and the load resistance, the
power to the load can be computed.
PULSE WIDTH
+30V
AVERAGE
VOLTAGE
15V 0'
+30V
2V 0'
ULTLfL rCRJUTJL
. ;JinnnnrL
ji n_
' ■ i i
JUUUL
(A) VARIA TION IN WIDTH WITH
CONSTANT PERIOD
(B) VARIATION IN PERIOD
WITH CONSTANT WIDTH
Fig. 10-4 Comparison of Pulse Width and Frequency Variation
68
MOTOR CONTROLS
EXPERIMENT 10 DIGITAL MOTOR CONTROL
MATERIALS
1 Oscilloscope
1 Multimeter
1 DC motor, 28 volt
1 DC power supply, 0-40V
1 NPN power transistor
PROCEDURE
1. Construct the circuit shown in figure 10-5.
1 Pulse generator (30 volt pulse variable
frequency and width)
1 Resistor, 47ft
1 Rheostat, 15012, 15W
1 Stroboscope
PULSE
GENERATOR
47fi
AW-
30V —
M) 28V DC MOTOR
Fig. 10-5 Switching Mode Motor Control Circuit
2. Set the pulse generator for 1kHz pulses and 30V positive amplitude.
3. Record and plot the average voltage across the motor as the pulse width is varied from
zero to 1000 jus. Also record and plot motor speed versus pulse width.
4. Set the pulse width to 500 jus.
5. Record and plot the average voltage across the motor as the pulse frequency is varied
from 50 Hz to 2kHz. Also record and plot motor speed versus pulse frequency.
6. Construct the circuit shown in figure 10-6.
7. Measure and plot the motor voltage, speed, power, resistance of R, and power of R as the
value of R is changed.
30V
28V DC
MOTOR
Fig. 10-6 Motor Control By Series Resistance
69
EXPERIMENT 10 DIG I TA L MO TOR CONTROL
MOTOR CONTROLS
Pulse Width Control
Pulse Width
CO
Pulse Frequency Control
Pulse Freq,
j
m
CO
DATA FROM STEP 3
DATA FROM STEP 5
Resistance Control
m
m
CO
DATA FROM STEP 7
Fig. 10-7 The Data Tables
70
MOTOR CONTROLS
EXPERIMENT 10 DIGITAL MOTOR CONTROL
ANALYSIS GUIDE. There are several advantages to digital, or switching mode, motor control
over analog control. One of these advantages that you should have become aware of in this ex-
periment is the efficiency and simplicity of the control circuit. Discuss how your results indi-
cated these advantages.
PROBLEMS
1. Did you find an advantage to either pulse width or pulse frequency variation over
the other? Explain your answer.
2. Make a graph showing the difference in efficiency between switching mode control
and resistance control for a range of motor voltages.
3. What would probably limit the frequency range of the pulses in a motor control?
4. What would probably happen as the pulses become very low in frequency?
71
experiment / / THE TRIAC FOR AC MOTOR CONTROL
INTRODUCTION. A fractional horsepower AC motor can be speed-controlled to some extent
by controlling the applied voltage. Although there are several different devices that work satis-
factorily, the triac makes possible one of the simplest circuits. In this exercise we will examine
the use of a triac for small motor control.
DISCUSSION. A triac is a three-element solid
state switch similar to the silicon-controlled
rectifier. The difference is that the triac can
be triggered into conduction in either direc-
tion. Triac is a generic term that was coined
to identify this triode semiconductor device.
Not only will the triac conduct in either direc-
tion, it can be triggered into conduction with
either a positive or negative pulse. Since the
device will conduct in either direction, there
is no cathode and anode; and the power ter-
minals are simply terminals T-j and J2- Ter-
minal T-| is the reference point for measure-
ment of voltages and currents. Figure 11-1
shows the physical structure, package, and
symbol for a typical triac.
The voltage and current characteristics
of the triac are very similar to those of an
SCR except they apply in both directions.
Figure 11-2 (A) is a typical characteristic
curve of an SCR showing forward and reverse
breakover voltage (Vbr) with different values
of gate current.
Figure 11-2 (B) shows the same curves
for a triac device. In quadrant one where T2
is positive, the curves are very similar to those
of the SCR. However, in quadrant three,
where T2 is negative, the triac has the same
curves as in quadrant one rather than just the
zener characteristic of the SCR. In either odd
quadrant, the gate current may be either posi-
tive or negative. Although the two quadrants
look the same, they may be a little different
for some triac units. Figure 11-3 (A) is a cir-
cuit using SCRs for full wave control of an
AC induction motor. Figure 11-3 (B) is the
GATE TERMINAL 1
(A) PELLET STRUCTURE (B) PACKAGE (C) SYMBOL
Fig. 11-1 The Triac Unit
I
MOTOR CONTROLS EXPERIMENT 1 1 TR I AC/AC MOTOR CONTROL
QUADRANT I T2 POSITIVE
(A) SCR (B) TRIAC
Fig. 1 1-2 SCR and Triac Curves
same control using a triac. The triac in figure with temperature change, by replacing R-|
11-3 (B) can be fired with a sharper pulse, with a device called a diac. The diac consists
which gives more stable operation especially of two four-layer diodes combined in the
(B) TRIAC CIRCUIT
Fig. 11-3 AC induction Motor Control Circuits
73
EXPERIMENT 1 1 TRI AC/AC MOTOR CONTROL
MOTOR CONTROLS
Fig. 11-4 Characteristics of the Diac
same manner as the triac, which is two SCRs
combined. The voltage current characteristics
of a diac are shown in figure 11-4.
The diac is nearly an open circuit until
a critical breakover voltage is reached. Then
it has a negative resistance characteristic until
a prescribed voltage drop is established across
it. This diac curve is very similar in shape to
a triac curve with no gate control. The break-
over voltage will vary, depending upon con-
struction of the unit, and cannot be changed.
In figure 11-5 the diac is in the gate cir-
cuit of the triac. When the capacitor charges
to the breakover voltage of the diac (which
may be about 6 volts), the diac very quickly
becomes a small voltage drop, discharging the
capacitor C with a surge of current into the
gate of the triac.
Although this circuit has a limited con-
trol range and a large hysteresis effect at the
low end of the range, it is small, simple, and
is suitable for many small-range applications
TRIAC
Fig. 11-5 Diac Used to Fire the Triac
74
MOTOR CONTROLS
EXPERIMENT 1 1 TRI AC/AC MOTOR CONTROL
Fig. 1 1-6 Fullwave Phase Control Circuit
such as light control, heat control, and fan
speed control.
The hysteresis effect or snap-back can
be observed when R p is increased until the
diac does not trigger and then reduced gradu-
ally. As Rp is reduced, the voltage across C
increases. When the diac triggering voltage is
reached, the diac discharges C to a lower volt-
age so that the trigger voltage in the opposite
direction on the next half cycle is reached
earlier in the half cycle. Therefore, as the
circuit begins to fire, it shifts in phase to an
earlier or smaller phase angle.
The operation of this circuit can be im-
proved considerably by adding three circuit
components which produce the circuit shown
in figure 11-6. The addition of C2, R2 and
R3, which is a second phase shift network,
extends the range of control and reduces the
hysteresis effect to a negligible amount. This
circuit will control the power in the load from
about 5 percent to about 95 percent of the
power that the load would draw direct from
the source. This circuit is, however, sensitive
to supply voltage variations and will shift in
operating phase with line voltage changes. The
circuit operates better when inductive loads
are connected between the two phase shift
networks, between R m and R3 in figure 1 1-6.
This provides line-referenced triggering at high
firing angles and triac-referenced triggering at
low firing angles. It helps reduce the unsy-
metric triggering caused by phase shift in the
load.
MATERIALS
1 OsciMoscope
1 Triac (Type 40723 or equivalent)
1 Diac (Type 1N5411 or equivalent)
1 AC Fan Motor, 110V AC
2 Capacitors, 0.1 /iF, 600W, DC
1 Resistor, 47kfi
1 Resistor, 68k£2
1 Variable Resistor, 250kl2,
linear taper, 1W
1 Stroboscope
75
txrtHIMENT 11 TRI AC/AC MOTOR CONTROL
PROCEDURE
1 . Construct the circuit shown in figure 1 1 -7.
0
MOTOR CONTROLS
FAN MOTOR
Jy^ 250 kn
115V AC
g 7
DIAC
^T*S 0.1
^TRIAC
Fig. 1 1-7 Simplified Triac Control Circuit
3. Construct the circuit shown in figure 11-8.
— 0 —
FAN MOTOR
R 3
68ktt
115V AC
J> 250kJ2
47kn
TRIAC
DIAC
0.1juF
0.1/iF
Fig. 11-8 Improved Triac Control Circuit
4. Take the appropriate data and plot the motor voltage, speed, and R, resistance versus the
triac firing angle. Show the hysteresis effect. 1 resiSTance versus the
76
MOTOR CONTROLS
EXPERIMENT 1 1 TRIAC/AC MOTOR CONTROL
First Circuit
Second Circuit
m
CO
m
CO
Fig. 1 1-9 The Data Table
ANALYSIS GUIDE. Because of the electrical characteristics of the triac, you should have recog-
nized some physical advantage of triac motor control circuits. You might also consider the cost
aspect of the control circuit. Discuss the operation of your control circuit and compare your
results to those you would get with SCRs.
PROBLEMS
1. How would you compare the operation of the two triac control circuits in this ex-
periment? Explain and illustrate.
2. What do you consider the main advantages of the triac? Be specific.
3. What might be the application of a triac with respect to DC motor control? Would
it have an advantage over an SCR? How?
4. Using a catalog determine the price of the control circuit in figure 11-8. How does
it compare to a typical SCR control of the same type?
77
experiment SYNCHRONOUS MOTOR CONTROL
INTRODUCTION. A synchronous motor has characteristics that are very useful in certain appli-
cations. Speed control of a synchronous motor is different from control of any other type of
motor. This experiment is an introduction to some of the characteristics and advantages of
synchronous motors.
DISCUSSION. A synchronous motor is an
AC motor operating from an AC voltage with
its speed of rotation a direct function of the
line voltage frequency and the motor's physi-
cal construction. The speed of an AC syn-
chronous motor is determined by the number
of its poles and the line frequency.
co = 120f/P or f=— X— = —
2 60 120
where co = Speed in RPM
f = Frequency in Hz
P = Number of poles
60 = Factor equating minutes
(RPM) and seconds (cps)
2 = Factor for AC excited poles
operating in pairs (one pole
operates the same as two
poles)
The construction of synchronous motors
varies considerably, but the general principles
are the same. Some examples of construction
will be presented to illustrate the principles of
operation, but actual construction may vary.
A synchronous motor consists of a stator (sta-
tionary poles) and a rotor (rotating poles) very
similar to other types of motors. One set of
poles has a constant field and does not change
with the AC line voltage.
These poles may be induced, permanent,
or excited by a DC current. Large motors
usually have DC-excited fields. The other set
of poles is excited by and will change with
the line voltage. It does not make any differ-
ence which set of poles rotates and which set
is stationary, but usually the constant field
poles rotate. For the purpose of this discus-
sion, the rotor will have the constant field,
and the stator will be excited by the line
voltage as shown in figure 12-1. The field
Fig. 72-7 Synchronous Motor Construction
78
MOTOR CONTROLS
EXPERIMENT 12 SYNCHRONOUS MOTOR CONTROL
<v hr K
90° 18 0° 270°
Fig. 12-2 Operation of the Synchronous Motor
created by the two AC-excited field coils can
be considered to be rotating, in either direc-
tion, in 180° steps. During the first half cycle,
one pole will be a magnetic north pole and
the other will be a magnetic south pole. Dur-
ing the next half cycle, the field has rotated
one-half turn because the poles have reversed.
Each succeeding half cycle will reverse the
poles, which represents a one-half revolution
of the rotating field. With two poles, the
field rotates one revolution for each cycle of
excitation voltage.
The reaction between the field and the
rotor is shown in figure 12-2. When the line
voltage is zero and about to start on the posi-
tive excursion, the north permanent pole is
just passing the pole that is about to become
a north pole. The same is true for the south
poles. As the excited poles become strong,
they repel the rotor. The rotor turns and, as
the permanent pole is midway between the
excited poles, their field is the strongest. As
the permanent north pole approaches the ex-
cited south poles, and vice versa, the excited
poles are decreasing in strength toward zero
as the poles pass. Then the excited poles re-
verse and increase, again keeping the rotor in
motion as long as it stays in step with the
field. If the rotor is stationary and the fields
are excited, it will start to rotate in first one
direction and then the other as the poles re-
verse. The rotor develops continuous torque
in one direction only when rotating in syn-
chronization with the field. As the load
changes, the rotor will shift in phase with the
rotating magnetic field but will stay in syn-
chronization, or cease producing torque and
stop.
Since the rotating magnetic field is mak-
ing exact half-tu.Ti steps, it could be consid-
ered to be rotating in either direction. The
rotor will operate equally well in either direc-
tion after it is started. Because it does not
produce torque out of synchronization, the
synchronous motor must be brought into
synchronization by some means. It can be
started by mechanically rotating the rotor to
proper speed. This may be done by another
motor, DC or nonsynchronous, either mechan-
ically linked or on the same shaft. Also, the
rotor may be modified to act as a synchronous
induction motor until proper speed is reached.
Low torque motors can be made to self-start
by shaping of the poles and the pole fields.
79
EXPERIMENT 12 SYNCHRONOUS MOTOR CONTROL
MOTOR CONTROLS
There are two distinct characteristics of
a synchronous motor. One, as mentioned
above, is the direct relationship between the
speed and the line frequency. The other is
the power factor characteristic. Large syn-
chronous motors can be made to operate with
a leading, zero, or lagging power factor by
changing the DC excitation to the permanent
fields. Conventional motors, because of their
inductance, cause a lagging power factor.
When adjusted to operate at a leading power
factor, the synchronous motor can improve
the overall power factor of a system that may
otherwise be operating with a lagging power
factor. Also, synchronous motors are often
more efficient than other types, especially in
the larger sizes.
Although the rotor of the synchronous
motor follows the rotating magnetic field,
relating frequency and speed, the instantane-
ous speed may fluctuate, meaning the rotor
will shift its phase relationship to the rotating
field. Often this type of motor tends to
"hunt", alternately leading and lagging the
rotating field. If the load is changed too
abruptly, the motor may hunt too much, slip
out of phase, and stall.
Small synchronous motors, such as those
used in clocks and control timers, do not have
any separate DC excitation. These motors are
usually hysteresis or reluctance types, but
some of them have permanent magnet rotors.
A small timing motor with a permanent mag-
net rotor will operate as previously described
but can be made self-starting. This is done by
shaping of the permanent and excited poles
and by the position relationship of the stator
and rotor poles along with a ratchet assembly
to keep it from starting in the wrong direction.
When the rotor of a split-phase motor
has properly-designed salient (shaped) poles,
it will start as an induction motor and run at
a synchronous speed. The rotor comes up to
nearly synchronous speed by induction-motor
action with a comparatively light load. As the
slip becomes negligibly small, the revolving
field permanently magnetizes the projecting
rotor poles. The rotor poles then "lock in
step" with the revolving fields of opposite
polarities and continue to rotate at synchro-
nous speed. This type of motor is called a
reluctance motor, getting its name from the
variable magnetic reluctance of the air gap.
When the number of salient poles on the rotor
is greater, by some multiple, than the number
of electrical poles on the statoc, the motor will
operate at a constant average speed that is a
submultiple of the apparent synchronous
speed and is called a subsynchronous reluc-
tance motor.
When the rotor of an induction motor
is built up of specially-hardened steel instead
of the usual silicon steel, the effect of hy-
steresis is greatly magnified. As a result, the
rotor will operate at synchronous speed be-
cause the hysteresis property of the rotor
strongly opposes any change in the magnetic
polarities once they are established. Many
electric clock motors operate on the hystere-
sis-motor principle. In the telechron design
in figure 12-3, a two-pole revolving field is
introduced into a sealed, thin metal cylinder
in which a shaft, carrying one or more hard-
ened magnetic steel discs, drives a gear train.
This motor is self-starting by the shaded pole
induction method.
A very popular method of construction
for timing motors is shown in figure 12-4.
The shape of the stator poles has the same
effect on start and direction of rotation of
the armature as the shaded poles in figure 12-3.
80
MOTOR CONTROLS
EXPERIMENT 12 SYNCHRONOUS MOTOR CONTROL
•HARDENED MAGNET STEEL DISKS
SHADING COIL-
CD
0
'EXCITING COIL
Fig. 12-3 Telechron Type of Hysteresis Motor for Operation of Clocks
STATOR
LAMINATIONS
ROTOR
SHAFT
STATOR POLES B' — '
VIEW B-B'-TOP PLATE
STATOR
WINDING
ALUMINUM
DISC
IRON RING
VIEW A-A'-ROTOR
Fig. 12-4 Construction of a Typical Synchronous Timing Motor
MATERIALS
1 Audio generator
1 Transistor Drive Transformer
(500ft CT - 200fi CT)
2 Power Transistors (type 2N3055
or equivalent)
2 Diodes (type 1N3639 or equivalent)
1 Transformer, 4:1, low side centertapped
1 DC power supply, 0-40V
1 Synchronous timing motor (1 15V, 60Hz)
1 Stroboscope
81
EXPERIMENT 12 SYNCHRONOUS MOTOR CONTROL
PROCEDURE
MOTOR CONTROLS
1 . Connect the synchronous motor to the 1 1 5V, 60 Hz line, and check the rotor speed.
2. Construct the circuit in figure 1 2-5.
NOTE: If PNP transistors are used, reverse the DC power supply polarity and
the diodes.
TRANSISTOR
DRIVER TRANSFORMER
AUDIO
OSCILLATOR
SYNCHRONOUS
MOTOR
Fig. 12-5 Circuit for Speed Control of Synchronous Motor
3. Set the DC power supply to 30 volts and the frequency of the audio oscillator to 60 Hz.
4. Increase the output level of the audio oscillator a little beyond the point at which the
motor begins to run smoothly.
5. Check the speed of the motor rotor and compare it with that of step one.
6. Record and plot the motor speed versus the oscillator frequency as the frequency is
changed from 30 Hz to 350 Hz, or as high as the motor will continue to run in synchro-
nization.
7. In the mid-range, change the frequency of the oscillator a little but quite abruptly and
observe how quickly the motor will attain the new speed, both faster and slower.
8. Count the poles and compute the speed for several different frequencies. Compare this
result to your frequency chart. Speed of motor when run from power line =
82
MOTOR CONTROLS
EXPERIMENT 12 SYNCHRONOUS MOTOR CONTROL
Frequency
Speed
Computed Speed
30 Hz
350 Hz
Fig. 12-6 The Data Table
ANALYSIS GUIDE. Many applications of synchronous motors rely on their speed character-
istics. Consider, along with its constant speed application, the possibility of controlling its speed
by a variable frequency supply. Discuss at least one variable speed synchronous motor appli-
cation.
PROBLEMS
1. How would you account for an error between your computed and measured speed
of a synchronous motor?
2. Was there a significant amount of slip in the synchronous motor? How much?
3. What are two applications of synchronous motors that are becoming very common
in most homes? Be specific. How many might be found in a typical home?
4. How do you account for your results in step 8?
83
experiment
13
INCREMENTAL MOTORS
INTRODUCTION. The incremental motor, or stepping motor, is different from other types of
motors It makes possible the simplification of control circuitry for precise mechanical position-
ing. This exercise deals with the basic principles and operation of incremental motors
DISCUSSION. Although the electrical step-
ping motor has been used since the early
1930s, it is only recently being considered for
more widespread use. One reason for the de-
lay in its use was because it is a digital device,
and only recently have digital computers and
other devices been developed for widespread
use. Industrial applications include batch
counting, process controls, and the accurate
positioning and actuating of machine tools.
Accuracies of machining or tool feeds to
0.001, 0.0025, or 0.0001 inch-per-step can be
achieved by proper mechanical drive mechan-
isms.
Types of motors other than incremental
are analog devices, because their speed or
amount of rotation is in some proportion to
the power input and varies along a continuous
scale. A specific amount of rotation cannot
be achieved without the aid of feedback. Even
with a digital or switching mode input, the
output is analog. In this case, the motor per-
forms as a digital to analog converter along
with the electrical to mechanical conversion.
The incremental motor is, however, a digital
motor. For each pulse, it rotates a discrete
step or specific amount, and it will rotate the
exact same amount for each pulse. The size
of the step depends on the construction of
the motor and on the motor controller. A
typical motor might have 10 or 12 steps per
revolution. The output-shaft rotation per step
may also be varied to achieve the desired sys-
tem response by the selection of appropriate
gear heads coupled to the motor.
Stepping motors are different from step-
ping switches or rotary solenoids in that the
motors are power devices. There are only
two basic types or principles of operation.
One is the rotary solenoid-ratchet type, with
mechanical "detenting" to hold it in position
between steps. The other type is the phase-
pulsed, synchronous-type motor. In general,
the solenoid type is simpler in concept and
easier to manufacture, while the phase-pulsed
is more complex in theory but simpler in
mechanization, which gives it several distinct
advantages. The solenoid-ratchet devices op-
erate by a solenoid stepping the armature one
step at a time with a ratchet mechanism. A
ratchet-pawl device is used for mechanical
detenting. The ratchet and detenting is the
key to the one-to-one correspondence between
pulses in and position out. Both physically
and analytically, this resembles local feedback.
Usually two solenoids are mounted in opposi-
tion for bidirectional rotation.
The phase-pulsed steppers are more near-
ly like other motors; in fact, certain types of
AC synchronous motors can be used as DC
stepping motors. Detenting (holding in posi-
tion between steps) is accomplished magneti-
cally, typically by interaction of the two
magnetic fields of a permanent magnet rotor
and a DC-excited field winding. Figure 13-1
shows a two-view cutaway sketch of a syn-
chronous motor with permanent magnetic
poles in the rotor.
84
MOTOR CONTROLS
EXPERIMENT 13 INCREMENTAL MOTORS
TEETH IN THESE SECTIONS
MUTUALLY OFFSET BY 1/2
ROTOR TOOTH PITCH
POLE WINDINGS
Fig. 13- 1 Sketch of Simplified Step Motor
The stator is made up of a two-phase,
four-pole winding with a total of eight poles,
and the rotor sections have ten teeth. To
make an efficient magnetic structure, the
rotor is made with two separate identical
discs, separated by a cylindrical permanent
magnet mounted so that one section is a north
pole and the other section is a south pole.
Proper magnet relationship is maintained by
having the two sections offset by one-half a
rotor tooth pitch. Figure 13-1 shows the
path of the DC flux field and the AC or pulse
flux field. Notice that the AC field does not
pass through the permanent magnet so it is
not demagnetized.
Figure 13-2 shows a four-step sequence
exciting the field poles to rotate the rotor.
Remember that the field poles operate in pairs
and that the two sections of the rotor are op-
posite magnetic poles and are offset physically
to align with the opposite field poles.
Characteristics vary with the motor, but
typical motors will precisely follow pulse
rates from 0 to 120 pulses per second and can
be slewed (driven faster but without the pre-
cision) at rates exceeding 200 pps. In very
gO
Fig. 13-2 Sequence Showing Rotating Flux
and Differential Indexing Action
special applications, synchronous stepping may
occur at rates up to 250 pps and slewing up
to 800 pps.
Special synchronous motors (enclosed
permanent-magnet types) can be used as slow-
speed AC motors or as incremental stepping
devices. The motor construction consists of
a permanent-magnet rotor and a two-phase
85
EXPERIMENT 13 INCREMENTAL MOTORS
MOTOR CONTROLS
Fig. 13-3 Switching Technique to Step a Synchronous Motor
stator winding. Output torque is produced
when current flows through the stator wind-
ings and acts on the permanent-magnet rotor.
Properly switching the DC voltage according
to figure 13-3 will cause the motor to step a
certain amount to a new position.
Bipolar motors can be used for DC step-
ping applications where the use of push-pull
circuitry or a center-tapped power supply is
not feasible. These motors have four wind-
ings rather than two, which allows the voltage
to be switched from one winding to another
in such a way as to give the effect of reversing
the current in one winding. Figure 13-4
shows the circuit connection and switching
sequence for stepping a four-winding motor.
The switching can be accomplished by me-
chanical switches, electromechanical relays,
or electronic devices operating directly from
a digital computer.
SWITCHING SEQUENCE
STEP
SWITCH NO. 1
SWITCH NO, 2
1
1
G
2
1
4
3
3
4
3
5
1 ,
1
5
Switching sequence for clockwise
rotation; reverse sequence for
count&rcfockwlse rotation.
SWITCH 2
Fig. 13-4 Switching Sequence for a Four-Winding Motor
86
MOTOR CONTROLS
EXPERIMENT 13 INCREMENTAL MOTORS
CW INPUT CHANNEL
CCW INPUT CHANNEL
MOTOR
TRANSLATOR
Fig, 13-5 Digital Control for Stepping Motor
Since an incremental motor is a digital
device, it is naturally frequently operated by
digital circuits. Figure 13-5 shows how a step-
ping motor is connected to digital circuitry
in an industrial numerical control machine.
Recent improvements in design and the
increase in digital equipment indicate that the
step motor will be used considerably more in
the future.
MATERIALS
1 Step motor, 28V
1 DC power supply, 0-40V
1 Power transistor (type 2N3055
or equivalent)
1 Audio generator
1 Capacitor, 10 /iF 50W, VDC
1 Diode (type 1 N3639 or equivalent)
1 Block and screw assembly with
mounting hardware
1 Set of pointers and dials for measuring
shaft rotation and block movement
1 Mechanical Breadboard
87
EXPERIMENT 13 INCREMENTAL MOTORS
MOTOR CONTROLS
PROCEDURE
1.
Set up system shown in figure 13-6.
a. Connect the step motor to the block and screw assembly.
Construct a scale to measure the block displacement.
Connect a switch so that actuation in one direction gives CW steps and in the other
direction gives CCW steps.
Construct a dial and indicator for the angular rotation of the shaft.
b.
c.
d.
SPDT
CENTER OFF
STEPPING MOTOR
POINTER
DIAL AND
POINTER
'FOR ANGULAR
MEASUREMENT
Fig. 13-6 Experimental Setup
2. Measure the number of steps per revolution of the shaft and the angular rotation per step.
3. Measure the relationship between the number of steps and the linear displacement of the
block. Use enough steps to minimize error in readings.
4. Measure the backlash (in steps and block movement) by approaching a certain detent
position from both directions.
5. Disconnect the motor from the block and screw assembly and set up the circuit shown in
figure 13-7.
AUDIO
OSCILLATOR
10 M F
-0
STEP MOTOR
©-
A
I
30V DC
<?
Fig. 13-7 The Experimental Circuit
88
MOTOR CONTROLS
EXPERIMENT 13 INCREMENTAL Motors
6. With the oscillator at 20 Hz, increase the output amplitude until the motor steps
cycle
each
7. Determine in RPM how fast the motor will step according to the oscillator frequency.
No. of
Steps
Shaft Rotation
(degrees)
No. of
Steps
Block Displ.
(mm)
Maximum stepping rate =_
RPM
Fig. 13-8 The Data Tables
ANALYSIS GUIDE. The stepping motor is normally used in applications which require accurate
repositioning. Explain why you feel that incremental positioning and repositioning would be
easier to control than analog positioning. Discuss what happens when you try to step an incre-
mental motor at too high a rate.
PROBLEMS
1. How can the problem of backlash be minimized in this type of position control?
2. How can the ratio of linear position to steps be changed? Name several methods
with their advantages and disadvantages.
3. Would there beany problem with multiple gear reduction? If so, what?
4. According to step 7, what might be a limitation of this type of stepping motor?
5. Show a block diagram and give appropriate details of how a standard motor could
be used for mechanical positioning to the same digital accuracy. Explain where
necessary.
89
experiment
TACHOMETER FEEDBACK
INTRODUCTION. Tachometer feedback is often used in motor control circuits for improved
system response. This exercise is intended to be an introduction to tachometer feedback
principles.
DISCUSSION. In many industrial applica-
tions it is essential that the speed of moving
materials or the rotational speed of revolving
equipment be kept relatively constant. As an
example, when the installation involves a
group of interconnected motor-driven sections
that must operate as a unit to move a continu-
ous sheet or strip of processed material, the
individual drives must be controlled through
some type of speed regulator that not only
coordinates the motion along the line to pre-
vent pile-up and excess tension, but maintains
a constant speed as well. In such mill opera-
tions as galvanizing and tin plating of sheet
steel or in the manufacture of paper, speed
regulators are often called upon to maintain
precise speed control, since the quality of the
manufactured product, i.e., thickness of plat-
ing or paper, is impaired if the speed is allowed
to fluctuate.
A common type of speed-sensing device
is the tachometer generator which is driven
INITIAL
by the drive motor and provides a feedback
signal to the motor control circuit. The ta-
chometer output voltage is connected so that
it is in opposition to the initial input signal.
The input to the control circuit is the alge-
braic sum of the two signals. A typical block
diagram is shown in figure 14-1.
An input voltage, E-|, is applied to the
input network. The output of the network,
E', is amplified by the amplifier which drives
the motor with a certain signal E 0 . Providing
the load is not too great, the motor will accel-
erate toward some maximum speed determined
by the amplifier output, motor characteristics,
and amount of load. The tachometer output
is proportional to the speed of its shaft, so as
the motor accelerates, the tachometer output
increases. As the tachometer output increases,
it opposes or cancels an increasingly larger
portion of the original input. Depending on
the output rating of the tachometer and the
design of the network, a speed will be reached
CONTROL
UT
INPUT
NETWORK
CONTROL
AMPLIFIER
to — SPEED IN RPM
MOTOR TACHOMETER
LOAD
E'
t> J KZ>-0-<D
FEEDBACK
Fig, 14-1 Basic Tachometer Feedback System
90
MOTOR CONTROLS
EXPERIMENT 14 TA CHOMETER FEEDBA CK
AMPLIFIER
MOTOR
TACHOMETER
LOAD 0-1 VOLTS/
REFERENCE
SIGNAL 100V
MOTOR & LOAD
+ AMPLIFIER
REF.
<*i REV/MIN
100(REV/MIN)/VOLT
10V
100 (REV/
MIN)/V
SPEED (
TACHOMETER
0.1V/(REV/
MIN)
Fig. 14-2 Simple Servosystem with Tachometer Feedback
that is stable. At this speed the tachometer
feedback, 0, will cancel a portion of the input
signal, E|. The resultant signal. E', is fed to
the amplifier and amplified to E 0 , which is
just enough to maintain the speed. When the
load is decreased allowing the motor to run
faster, the tachometer output increases, can-
celling a greater portion of the input E t 'leav-
ing the resultant signal E' and the amplified
signal E c less, keeping the speed very near its
original value. When the load is increased
slowing the motor, the tachometer output
decreases and allows a larger signal to be am-
plified, increasing the input to the motor and
keeping it very near the original speed. Of
course, the load cannot be increased beyond
the ratings of the motor and driving amplifier.
The overall response or speed stability of the
system will be determined by the design of
the system, including the network parameter,
gain of the amplifier, and characteristics of
the motor and tachometer.
The following example of calculating the
motor speed and error signal is for the system
shown in figure 14-2. The input network in
this circuit is just a simple series feedback
connection with no attenuation. The follow-
ing is an example of the mathematical analysis:
E' = Error Voltage
E' = Input - Feedback from tachometer
E' = E 1 - oj X tachometer volts/(rev/min)
E'= E 1 - co G
E' = 100 - 0.1 co volts differential input
to amplifier (14.1)
Amplifier output voltage = error voltage X am-
plifier gain:
E G =E'XA
E o = 10E'
(14.2)
Motor speed = amplifier output voltage times
the motor speed per applied volt:
co = E Q X M
co = AE' X M
and substituting equation 14.2
co= 10E' X 100
co= 1000E' RPM
(14.3)
91
EXPERIMENT 14 TA CHOMETER FEEDBA CK
MOTOR CONTROLS
By substituting equation 14.3 into equation
14.1,
E'= 100 - 0.1 X 1000E'
E'= 100- 100E'
101 E'= 100
E' = 0.9901
And now substituting back into equation 14.3,
co = 1000 E'
co = 990.1 RPM
Now if we go back and combine equations
«8> — D>
S FB
14.1 and 14.3 we can get an equation for the
system:
E'= - Geo and co = AE'M
co - AM (E 1 - Geo)
co = AME 1 - AMGco
co + AMGco = AME 1
co =
AM \
^1 + AMG/
(14.4)
If we consider AM in this circuit analo-
gous to the amplification (A) for an amplifier
and G to the feedback ratio, this equation is
the same as that for finding the gain of a
feedback amplifier as shown in figure 14-3.
>^<iin>-r
LOAD
T
I
I
I
I
I
w = E
( AM \
I VI + AMG /
=OUT
E OUT " E 1
Fig, 14-3 Gain Equations for Closed Loop Systems
92
MOTOR CONTROLS
EXPERIMENT 14 TACHOMETER FEEDBACK
REFERENCE
VOLTAGE
100V
100 V
ERROR e
TIME
TRANSIENT
ERIOD
PEI
STEADY^TATE
'PERIOD
J ^+ 0.9901V
1 TIME
SHAFT
SPEED N
REFERENCE SPEED
1000REV/MIN
SPEED ERROR
9.9 REV/MIN
^ TRANSIENT |
STEADY-STATc
Tperiod r
PERIOD
TIME
Fig. 14-4 The Response of the System in Fig. 14-2 to a Step Change in Input Signal
The response for the system in figure
14-2, which is assumed to have some over-
shoots during the transient period, is shown
in figure 14-4.
Position controllers differ from speed
regulators in that no error exists under nor-
mal operating conditions. A simple remote
position control servosystem is shown in fig-
ure 14-5. The input and output potentiom-
eters form a bridge, with the bridge output
being the amplifier input. When the input
potentiometer is moved to a new position,
the bridge is unbalanced, providing an input
to the amplifier which drives the motor in the
proper direction to bring the bridge back into
balance.
DC
SUPPLY
ERROR e = V 1 - V 2
AMPLIFIER MOTOR
LOAD
— )
OUTPUT
POTENTIO-
METER (TRANS-
DUCER)
Fig. 14-5 Simple Remote Position Control Servosysti
93
em
EXPERIMENT 14 TACHOMETER FEEDBACK
MOTOR CONTROLS
(a) INPUT
(b) OUTPUT &2
REQUIRED INPUT
<c> (R)
(d) ERROR
B 2
(e) ADDITIONAL INPUT
I = R-ERROR
(f) OUTPUT DERIVATIVE
OF VELOCITY
(de 2 /dt)
Fig. 14-6 Waveform of Response of an Overdamped System to a Step Input,
Showing Required Stabilizing Signal
If a step input, 0], is caused by an
abrupt change of the input potentiometer,
figure 14-6(a), and an overdamped response
is required (no overshoot) as shown in figure
14-6(b), the amplifier input has to be modi-
fied. The load must be accelerated between
A and B, and decelerated between B and C,
to stop at the desired position. The required
input, R, shown in figure 14-6(c) is large and
positive initially to provide torque to set the
load in motion. It decreases to zero at B
where it goes negative to decelerate the load
to a stop at the desired position without over-
shooting. The position error input to the am-
plifier is shown in figure 14-6(d) and is the
difference between the input and output po-
tentiometers. The additional input needed is
the difference between the required signal fig-
ure 14-6(c) and the error signal (figure 14-
6(d) ).\
Requi/ed input (R) = error + additional
- input (I)
I = R - error
The waveform that satisfies the above equa-
tion is shown in figure 14-6(e).
The graph of the output shaft velocity,
which is the derivative (slope) of the output
94
MOTOR CONTROLS
EXPERIMENT 14 TACHOMETER FEEDBACK
OUTPUT
POTENTIOMETER
(TRANSDUCER)
Fig. 14-7 Remote Position Control Servosystem with Tachometer Feedback
position curve is shown in figure 14-6(f). This
curve is the right shape but the wrong polarity.
If the polarity of this curve is reversed and
subtracted from the input error signal, the
proper waveform will be achieved. The out-
MATERIALS
1 DC motor, 28V, DC 1/100 HP
at 7000 RPM
1 DC tachometer generator, approxi-
mately 3V DC/1000 RPM
1 DC power supply (0-40V)
1 Multimeter
1 Stroboscope
1 Set of mounting hardware for
motor-tachometer drive
1 Transistor, 2N3819 or equiv.
put of a tachometer generator is directly pro-
portional to the speed of rotation of the rotor,
so it will give the additional signal needed
when properly connected into the circuit as
shown in figure 14-7.
1 Transistor, 2N268 or equiv.
3 Potentiometers, 1 meg.fi, 1/2W
1 Circuit board
2 Resistors, 1 megfi, 1/2W
1 Resistor, 100fi, 1/2W
1 Resistor, 1kfi, 1/2W
1 Resistor, 5kfi, 1/2W
1 Resistor, 3fi, 10W
PROCEDURE
1- Take data and plot the RPM versus volts graph of the motor by applying eight or ten
different values of voltage and checking the speed.
2. Take data and plot the RPM versus volts
voltage at eight or ten different speeds.
output for the tachometer by measuring the
95
EXPERIMENT 14 TACHOMETER FEEDBACK
MOTOR CONTROLS
3. Construct the circuit shown in figure 14-8,
© +28VDC
Fig. 14-8 Servo with Tachometer Feedback
4. Measure the DC gain of the amplifier by plotting an input versus output voltage graph.
5. Compute the stable RPM of the system. Show all calculations.
6. Measure the stable RPM of the system and compare the results with step five.
Motor Data
V
m
CO
Generator Data
CO
Amplifier Data
V:
Computed Stable RPM of system =
Measured Stable RPM of system = ,
Fig. 14-9 The Data Table
96
MOTOR CONTROLS EXPERIMENT 14 TACHOMETER FEEDBACK
ANALYSIS GUIDE. Tachometer feedback is frequently used in many types of control circuits
Because of the nature of this feedback, it is used to improve the operation of control systems in
several ways Discuss the extent to which your system effectively regulated the motor speed
Discuss any difficulty that you encountered.
PROBLEMS
1. What does the mechanical linkage between the motor and the tachometer have to
do with the equation for the loop gain?
2. Would this type of system help to keep the motor at its maximum rated RPM or
torque? Explain.
3. Compare the function of the tachometer in a rate and a position system.
4. Will a gear ratio in the mechanical linkage between motor and generator affect the
system? How?
97
exoertmen t SERVOMECHANISMS
INTRODUCTION. The subject of servomechanisms, from either a theoretical or a practical
point of view, is an extensive and interesting study. This experiment is intended as a brief intro-
duction to some of the basic concepts and applications of elementary servomechanisms.
DISCUSSION. A servomechanism (servo) is a
machine or mechanism designed to carry out
orders. An example of a basic servo is shown
in figure 15-1 and contains a DC motor, two
batteries, and a switch.
When the switch is in one position, the
motor will rotate in one direction: when the
switch is in the other position, the motor will
rotate in the opposite direction. This circuit
constitutes a simple servo. By using large
motors and high power amplifiers or motor
control circuits along with proper feedback,
large jobs can be precisely done. Although
the circuit in figure 15-1 is actually a servo
because it responds to signals telling the motor
which way to run, a true servo must not only
control direction, it must also control the
amount and/or speed of rotation.
There are many control systems that do
not require extreme accuracy and are used
open-loop. Many motor speed control cir-
cuits are open-loop systems. The open-loop
system responds to a control signal to do a
specific function, but there is no feedback
from the output function to the input to in-
sure that the output has responded satisfac-
torily. A basic block diagram of a servo
mechanism with feedback is shown in figure
15-2. These systems usually operate with
only negative feedback and in this discussion,
feedback is understood to be negative.
i — H|
Fig. J 5-1 A Basic Servo
98
MOTOR CONTROLS
EXPERIMENT 15 SE R VOME CHA NISMS
ERROR
I CONTKOL
I
Fig, 15-2 Block Diagram of a Servo with Feedback
A servomechanism is a closed-loop sys-
tem in which the output, or a portion of it, is
fed back for purposes of comparison with the
input, and the difference between the two is
amplified and fed to an actuator so that the
output follows variations of the input. Using
servomechanisms it is possible to control
automatically a vast number of physical quan-
tities. This ability to control an almost un-
limited variety of physical quantities has been
made possible through a reduction of the
problem of automatic control to its basic or
fundamental elements, known as the general-
ized servomechanism.
The closed-loop circuits may be con-
sidered as having certain common character-
istics in that they all possess certain common
elements. Insight into their common char-
acteristics is a relatively recent development.
Even though the ancient Egyptians knew about
automatic lighting or transferring devices and
James Watt developed a governor to maintain
a constant speed for his steam engine, which
was a typical servomechanism, an analysis of
automatically-steered bodies published in 1922
is credited with containing the first insights
that led to contemporary servomechanism
theory. Later, in 1932, an analysis of feed-
back in electronic amplifiers provided general
insights into the dynamic characteristics of
control systems. World War II caused a de-
mand for automatic control of radar antennas
and guns which increased the general dissemi-
nation and study of the properties of these
systems.
99
EXPERIMENT 15 SERVOMECHANISMS
MOTOR CONTROLS
B inO
Gtein-fcout'
/
FEEDBACK
NETWORK
c out
ERROR
DETECTOR
o-
COMMAND OR
REFERENCE
OR SIGNAL
INPUT
e C e in-0 e out
(At FEEDBACK AMPLIFIER
ACTUATOR DAMPER
CONTROLLER
GAIN = G
0<<W>
TRANSDUCER
♦
I
I
►O out
LOAD
CONTROLLED
OUTPUT
(B) GENERALIZED SERVOMECHANISM
Fig. 15-3
The similarity between the feedback am-
plifier and the generalized servomechanism is
shown in figure 15-3. The amplifier circuit
in figure 15-3 (A) has a portion of the output
fed back to the input through a feedback
network.
The feedback factor (3 is the ratio of the
feedback to the output. The feedback signal,
j3 times the output, is fed back in opposition
to, or out of phase with, the input signal (neg-
ative feedback) in order to obtain the desired
results. Letting G represent the gain of the
amplifier and 0 the feedback factor, the fol-
lowing relationships exist:
1 - j3G > 0 negative feedback exists
1 - j3G < 0 positive feedback exists
1 - jSG = 0 the system will oscillate
100
MOTOR CONTROLS
EXPERIMENT 15 SE R VOME CHA NlSMS
In figure 15-3 (A) the input to the am-
plifier is the difference between the input
e in and the feedback j3e out , or Error = e jn -
/3e Qut . The output of the amplifier will then
be the gain of the amplifier times the input to
the amplifier, or e Qut = G X error. If for
some reason the output tended to become
too high, the feedback increases, thus decreas-
ing the input to the amplifier bringing the
output back to normal. In the same manner,
if the output is low the feedback decreases
which cancels less of the input or allows a
larger signal to the amplifier, bringing the
output back up to normal. The ratio of out-
put to input for the system, e out /e jn , is the
closed-loop gain of the system and can be
found by combining the two expressions
above for e Qut and error voltage.
Error = ej n -/te,
B out
and
c out
= G Error
Combined, this gives
B out
= G < e in-#W
Solving for e Qut gives
e out = Ge in " G ^ e out
e out
G ^ e out = Ge in
e 0 ut< 1 +Gj3) = e in G
e out e in \ iL +
The feedback amplifier of figure 15-3 (A)
is a specific case of the more generalized servo-
mechanism shown in figure 15-3 (B). The
primary function of the negative feedback is
to give a constant output with a constant in-
put regardless of such variables as temperature
and load changes. Other advantages of nega-
tive feedback are improved frequency response
or response time, reduced distortion, and
reduced noise. Figure 15-4 is a block diagram
of a basic servo system that could operate
with almost any units or quantities.
Q
= quantity of input function
= quantity of output function
= transfer ratio of the input function
transducer
= transfer ratio of the output
function transducer
= transfer ratio of the feedback
function transducer
= input signal
= output signal
= gain of signal amplifier
= feedback factor
Error = actual signal input to controller
amplifier
J in
s o
G
0
INPUT
V ERROR
CONTROLLER
AMPLIFIER
GAIN = G
To
LOAD
i
Fig. 15-4 Block Diagram of a General Servomechanism
101
EXPERIMENT 15 SE R VOME CHA NISMS
MOTOR CONTROLS
The generalized servomechanism may be
used to control a number of physical quanti-
ties, such as displacement, pressure, velocity,
force, temperature, etc., in addition to electri-
cal quantities such as current and voltage.
Since the electronic voltage or current
amplifier is probably most commonly used in
servomechanisms, the input is considered to
be a voltage input. This method of control
involves the use of one transducer or actuator
which converts the output of the amplifier
to some quantity of a function, and another
transducer which converts the controlled out-
put quantity to an electric voltage for feed-
back. A similar transducer may be used for
providing the input function.
As in the case of the feedback amplifier,
the closed-loop gain of the system is expressed
as
rotations for the feedback amplifier and the
servomechanism is shown below.
/"\lll|Jlll Id
^Prvnmpphsinicni
uCI VUIIICUI lul II 31 11
e in
u i'i b in
e out
S o T o = Q o
P
G
G
The equation for the closed-loop gain of
the servomechanism can be derived the same
way as the gain of the feedback amplifier.
By using the amplifier equation,
closed-loop system gain
output quantity
input quantity
A direct comparison can be made be-
tween the voltage amplifier and the servo-
mechanism.
The quantity of input function, Qj, times
the transfer function, Tj, of the input trans-
ducer will give the input signal, Sj n , which
compares to ej n for the voltage amplifier.
The gain of the controller amplifier, G, and
the transfer function of the output transducer,
T Q , compares to the gain of the voltage am-
plifier. To find the quantity of output func-
tion, Q Q/ the error is multiplied by the gain
of the controller amplifier, G, which gives
Q Q = (Error)GT Q . In this circuit the feedback
is 0 times the transfer function Tp of the
feedback transducer. A comparison of the
and substituting the functions from the chart,
the gain equation for the servomechanism is
If the amplifier operates with the same energy
form at the input and output, a transducer for
that function is not needed. This general ex-
pression for the gain of a closed loop system
should apply for any particular system by
substituting the specific functions into the
general equation.
One other point that was not brought
out previously is that there may be a mechan-
ical linkage change between the output trans-
ducer and the feedback transducer. This
linkage change, such as a gear ratio, must be
considered in the feedback expression for the
system.
102
MOTOR CONTROLS
EXPERIMENT 15 SE R VOME CHA NISMS
MATERIALS
1 DC Motor, 28V
1 Tachometer generator (approximately
3V DC/1000 RPM)
1 Set of hardware for mounting and
coupling motor and generator
1 Chart recorder or oscilloscope
1 Switch SPST
3 Potentiometers, 1 meg£2, 1/2W
2 Resistors, 1 meg£2, 1/2W
PROCEDURE
1 Resistor, 5k£2, 1/2W
1 Resistor, 1kfi, 1/2W
1 Resistor, 10012, 1/2W
1 Transistor, FET 2N3819 or equivalent
1 Transistor, 2N268 or equivalent
1 Resistor 3£2, 10W
1 Circuit board
1 Multimeter
1 DC power supply, 0 - 40V
1. Assemble the servo mechanism shown in figure 15-5.
2. Set the gain control for maximum gain and set the feedback control for mini
feedback.
minimum
3. With St closed, adjust the input for a certain speed and the response recorder for a nomi
nal reading. Then open .
4. Close S,. Observe and compare the response time of the system at the tachometer
output and at the drive motor input.
+28VDC
CHART
RECORDER
OR
OSCILLOSCOPE
Fig. 15-5 Servo System for Speed Control
103
EXPERIMENT 15 SE R VOME CHA NISMS
MOTOR CONTROLS
ANALYSIS GUIDE. The servomechanism is a very important part of electromechanical tech-
nology. Understanding the operation and how to modify the response of servomechanism is a
key to understanding electromechanical systems. Compare each of your responses to each other
and discuss how feedback affects the results.
PROBLEMS
1. How will the gain of the amplifier affect the operation of the servomechanism? Is
the gain value critical?
2. What would probably be the limiting factor in how fast a servomechanism could
respond to an input signal?
3. What is the difference in operation of a system with input, output, and feedback,
and a small servomechanism that is a component in the larger system? Explain.
104
EXPERIMENT 1 Name
Date: Class Instructor
'FO
'g
V CE
R on
■h
Fig. 1-11
The Data Table
EXPERIMENT 2
♦
Date:
Name
Class
Instructor
Condittons
1^
v m
CO
Start
6^ = 0
Run
Fig. 2-7
The Data Table
EXPERIMENT 3
Date:
Name ,
Class m Instructor
CO
m
CO
Fig. 3-13 The Data Tables
EXPERIMENT 4
Date:
Name
Class
Instructor
Condition
DC
bias
5V AC
bias
10V AC
bias
1 R\/ AP
I O V ML
bias
Start
Run
^.4-5 Values of/,
EXPERIMENT 5
Date:
Name
Class
Instructor
dv/dt
(approx.)
R
(approx.)
C=1.0mF
C = 0.1 nF
e
V
6
V
Fig. 5- 1 1 The Data Tables
EXPERIMENT 6
Date:
Name
Class
Data from: Fig. 6-8
Fig. 6-9
CO
(RPM)
CO
(RPM)
Fig. 6-10 Fig. 6-11
Fig. 6-12 The Data Tables
EXPERIMENT 7
Date:
Name
Class
Instructor
V BE
V EE
1=0,
. = _E
V BB
V EE
= 0,
= +E
V
V
BB = 5V «
EE = + E
V BB = 1 0V f
V EE = +E
V BB = 20V,
v ee = +e
V E
'E
V E
'e
V E
'e
'b2
V E
'e
•b2
V E
'E
'b2
:
Fig. 7-5 The Data Table
EXPERIMENT 8
Date:
Name
Class
Instructor
V
m
CO
V
m
1
Results, Figure 8-8 Resu/tSf Figure 8 _ JQ
Fig. 8-12 The Data Tables
EXPERIMENT 9
Date:
Name
Class
T
= 250)
iS
T
= 500/
JLS
T
= 750 ms
T
= 900 /is
V P
l Vm
CO
V P
V m
CO
v p
CO
V P
V m
CO
Fig. 9-8 The Data Table
EXPERIMENT 10
Date:
Name
Class
Instructor
Pulse Width Control
Pulse Width
V
m
DATA FROM STEP 3
Pulse Frequency Control
Pulse Freq.
V
m
DATA FROM STEP 5
Resistance Control
m
V
m
CO
DATA FROM STEP 7
Fig. 10-7 The Data Tables
EXPERIMENT 11
Date:
First Circuit
V
m
CO
Second Circuit
m
CO
— ' 1 l_
Fig. 11-9 The Data Table
EXPERIMENT 12
Date:
Name
Class
Frequency
Speed
Computed Speed
30 Hz
350 Hz
Fig. 12-6 The Data Table
EXPERIMENT 13
Date:
Name
Class
Instructor
No. of
Steps
Shaft Rotation
(degrees)
No. of
Steps
Block Dispi.
(mm)
Maximum stepping rate = RPM
Fig. 13-8 The Data Tables
EXPERIMENT 14
Date:
Motor Data
m
CO
Generator Data
a?
Computed Stable RPM of system =
Measured Stable RPM of system =
Fig. 14-9 The Data Table
Amplifier Data
V:
EXPERIMENT 15 Nam e
11/71 (1C 141)
ff
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